ARRANGEMENT FOR ADJUSTING THE NATURAL CONVECTION AIRFLOW IN COMBINED REFRIGERATION APPLIANCES Field of the Invention The present invention refers to a constructive arrangement applied to combined refrigeration appliances of the type comprising a refrigerating compartment and a freezing compartment separated from each other, closed by respective front doors and each lodging, internally and in the rear portion thereof, a respective evaporating plate portion responsible for cooling the airflow which circulates by natural convection inside the respective refrigerating or freezing compartment. Prior Art Combined refrigeration appliances comprising a cabinet defining therewithin a freezing compartment and a refrigerating compartment, which are generally superposed, separated by a horizontal dividing wall and closed by respective front doors, are well known in the prior art. In these combined refrigeration appliances the refrigerating and freezing compartments are each provided with a respective evaporating plate portion. In the refrigerating compartment, said evaporating plate portion is usually vertically disposed and secured against a rear wall portion of the respective compartment . These evaporating plate portions are dimensioned to refrigerate the descending airflow produced by natural convection in the interior of the respective refrigerating and freezing compartments and are disposed in series in the same refrigerating circuit so as to receive the already condensed refrigerant fluid being pumped by a compressor of said refrigerating circuit. Generally in these combined refrigeration appliances the control of the temperature and consequently of the refrigeration of the refrigerating compartment is effected by the adjustment of a thermostat (when mechanical) , or by an electronic system, generally using a control module of the refrigeration appliance. In many of these combined refrigeration appliances both the refrigerating and the freezing compartments are refrigerated even though one of the compartments does not require further refrigeration. In these constructions, the refrigeration appliance does not have independent devices to control the refrigeration capacity of each of said refrigerating and freezing compartments, which can cause serious operational problems . When the freezing compartment of these combined refrigeration appliances is submitted to a severe loading condition and/or the ambient temperature is very low, the compressor can be maintained switched on for a longer period than the refrigerating compartment needs, which can reduce the air temperature therein to negative or near zero values and freeze or excessively refrigerate the food stored therein, which is not desired to occur inside the refrigerating compartment. This type of problem may occur due to the fact that the temperature control of both the freezing compartment and the refrigerating compartment is carried out only by a temperature sensor or by a thermostat usually positioned in the evaporating plate portion of the refrigerating compartment. When the freezing compartment is loaded, the thermal load applied to the refrigeration system is consequently increased, raising the evaporation temperature common to both compartments and avoiding the evaporating plate portion of the refrigerating compartment from reaching, in a determined time interval, a temperature which is sufficiently low to provoke the switching off of the compressor. Since this temperature for a two-door combined refrigeration appliance is normally equal to or lower than -2O0C, the temperature of the evaporating plate portion of the refrigerating compartment will be the same due to the freezing temperature required from the evaporating plate portion designed for the freezing compartment. Since the temperature difference between the evaporating plate and the air temperature in the refrigerating compartment is significantly higher than that in the freezing compartment, the refrigerating compartment will experience excessive refrigeration, since its evaporating plate portion, which is also submitted to a refrigeration of about -2O0C, will provoke excessive cooling of the descending airflow which is forced to pass there along by natural convection inside the refrigerating compartment. The excessive refrigeration of the refrigerating compartment can also cause problems to the food stored therein. A possible solution for this problem is to reduce the evaporating plate portion of the refrigerating compartment. Nevertheless, such reduction can cause a problem of insufficient refrigeration when a reverse situation occurs, i.e., when the refrigerating compartment is severely loaded. In this case, a smaller evaporating plate portion results in a thermal exchange area which is insufficient to comply with the refrigeration requirements determined by the user upon loading the refrigerating compartment with a quantity of food products to be refrigerated. Other possible situation in which a similar problem can occur is when the ambient temperature is low, i.e., when the refrigeration appliance is used in cold regions. In this condition, the thermal load applied to the appliance is highly reduced and since the evaporation temperature is usually significantly lower than the ambient temperature, the refrigeration appliance still maintains a certain temperature difference in relation to the external ambient, which can cause excessive refrigeration. Objects of the Invention As a function of the problems discussed above, it is a generic object of the present invention to provide an arrangement for adjusting the natural convection airflow inside the refrigerating compartment of a two- door combined refrigeration appliance which makes use of the natural convection system to produce circulation of air inside its refrigerating and freezing compartments. It is a more specific object of the present invention to provide an arrangement for adjusting the natural convection airflow in combined refrigeration appliances such as mentioned above, which allows the user to adjust the refrigeration capacity in the refrigerating compartment only when necessary, making said refrigeration capacity adequate to the thermal load existing in said compartment so as to refrigerate the stored food, without freezing or excessively reducing the temperature inside said compartment even when the freezing compartment is requiring a more intense operation from the refrigeration appliance. Summary of the Invention Aiming at reaching the objects mentioned above, the arrangement of the present invention comprises a deflecting plate, which can be or not made of a thermal conductive material, such as a metallic sheet, which is substantially vertically disposed in front of the evaporating plate portion of the refrigerating compartment, so as to occupy the width of the latter and to define, with a rear wall of said refrigerating compartment, an air cooling chamber lodging said evaporating plate portion and presenting an air inlet upper opening and an air outlet lower opening, communicating the air cooling chamber with the interior of the refrigerating compartment. One of said upper and lower openings is provided with an obturator which can be displaced between a closed position, in which it blocks the natural convection airflow descending through the air cooling chamber, and at least one open position in which it allows said natural convection airflow to descend through the air cooling chamber. The construction proposed above allows the user to activate the obturator in order to totally block, partially restrain, or totally release the airflow descending through the air cooling chamber and consequently along the evaporating plate portion of the refrigerating compartment, so as to adjust the refrigeration degree inside the refrigerating compartment, independently of the requirements of the thermal load stored inside the freezing compartment, i.e., independently of the operating time of the compressor of the refrigeration circuit of the combined refrigeration appliance. The construction proposed herein further allows the user to adjust the refrigeration capacity inside the refrigerating compartment as a function of the characteristics of the operation required from the appliance. The user can, for example, reduce said refrigeration capacity during winter to prevent the refrigerating compartment from working in a very cold condition, or increase the refrigeration capacity during summer, when the appliance works with higher thermal loads. By reason of the procedures proposed herein, the combined refrigeration appliance is provided with an evaporating plate portion of the refrigerating compartment which is dimensioned to comply with a higher thermal load, even in case said evaporating plate portion works over-dimensioned during determined operating phases of the refrigeration appliance. For example, in winter, the user may adjust the obturator to reduce the airflow through the air cooling chamber, reducing the refrigeration capacity inside the refrigerating compartment, even with said evaporating plate portion being dimensioned to produce a thermal exchange with a much more intense airflow and corresponding to a totally open obturator position. Brief Description of the Drawings The invention will be described below, with reference to the enclosed drawings given by way of example of possible constructive arrangements, and in which: Figure 1 schematically illustrates a longitudinal vertical sectional view of a combined refrigeration appliance of the type considered herein and presenting a first embodiment of the present arrangement; Figure 2 is an elevational front view of the upper portion of the refrigerating compartment, illustrating the relative positioning of the deflecting plate in relation to the width of said refrigerating compartment; Figure 3 is an enlarged vertical sectional view of the upper portion of the refrigerating compartment, illustrating the assembly of the obturator in the deflecting plate, said section being taken according to line III-III in figure 2; Figures 4 and 4a are enlarged sectional views of the obturator assembly of figure 3 in the closed and open positions, respectively; Figure 5 is an exploded rear perspective view of both the deflecting plate and the obturator; and Figure 6 is a view similar to that of figure 1, but illustrating another embodiment for the positioning of the obturator of the present arrangement . Detailed Description of the Invention As previously mentioned and illustrated in the appended drawings, the present arrangement is applied to a combined refrigeration appliance of the type which comprises a cabinet 10 constructed in any adequate manner and internally forming a refrigerating compartment R and a freezing compartment F separated from each other by a horizontal dividing wall 11, closed by respective front doors 12 and 13 and lodging, internally and in the rear portion thereof, a respective evaporating plate portion 20, 30, which is vertically disposed and secured to the cabinet, maintaining a certain distance in relation to an adjacent portion of a rear wall 14 of the latter. As illustrated in the drawings, the refrigerating compartment R is delimited by the rear wall 14, superiorly by the dividing wall 11 of the cabinet 10, by the respective front door 13, by a pair of lateral walls 15 and by a bottom wall 16. According to the invention, there is provided a deflecting plate 40, generally made of a metallic plate of high thermal conductivity or other material such as plastic, which is substantially vertically secured inside the refrigerating compartment R, parallel to and in front of the evaporating plate portion 30 of said refrigerating compartment R. The deflecting plate 40 is preferably dimensioned to occupy practically the whole width of the refrigerating compartment R, defining with the rear wall 14 of the latter an air cooling chamber 50 in which the respective evaporating plate portion 30 is lodged. The air cooling chamber 50 presents an air inlet upper opening 51 and an air outlet lower opening 52, said upper opening 51 and lower opening 52 communicating the air cooling chamber 50 with the interior of the refrigerating compartment R. Still in accordance with the invention, one of said upper opening 51 and lower opening 52 is provided with an obturator 50 that can be displaced between a closed position, in which it blocks the natural convection airflow descending through the air cooling chamber 50, and at least one open or partially open position, in which it allows a corresponding airflow to descend through the interior of the air cooling chamber 50 and along the respective evaporating plate portion 30. In the embodiment illustrated in figures 1 and 2, the deflecting plate 40 presents an upper edge 41 seated against the adjacent portion of the dividing wall 11 of the cabinet 10, and in this case the air inlet upper opening 51 in the air cooling chamber 50 is defined by at least one window 40a or a respective plurality of windows 40a provided in the deflecting plate 40 itself and the obturator 60 is mounted in said upper opening 51 of the windows 40a. In this construction, the deflecting plate 40 presents a lower edge 42 which is maintained spaced in relation to the adjacent portion of the rear wall 14 of the refrigerating compartment R, defining the air outlet lower opening 52 of the air cooling chamber. In the embodiment illustrated in figures 1-3, the air outlet lower opening 52 of the air cooling chamber 50 is defined by a plurality of openings 70a produced in the finishing plate 70 which is molded preferably in plastic material and provided with rear fixation means 71 to be fitted in respective housings provided in any of the parts of evaporating plate portion 30, deflecting plate 40 or even the internal wall of the cabinet of the combined refrigeration appliance. Still according to the embodiment illustrated in figures 1-3, the obturator 60 takes preferably the form of an elongated plate, preferably made of a synthetic material and dimensioned so as to completely cover the upper opening 51 and the respective air inlet windows 40a in the air cooling chamber 50. The obturator 60 is provided with a plurality of orifices 60a disposed according to an alignment coinciding with that of the window 40a of the deflecting plate 40 and which are dimensioned to coincide with the respective windows 40a when the obturator 60 is displaced to the full open position illustrated in figure 4a of the enclosed drawings. The obturator 60 in the illustrated construction incorporates a pair of small rear lower teeth 61 and a rear upper jaw 62 which are respectively slidingly fitted in respective slots 45 and 46 provided in the deflecting plate 40. With this construction, the user needs only to promote the linear displacement of the obturator 60 between a closed position, illustrated in figure 4, in which its orifices 60a are displaced out from the alignment with the windows 40a of the deflecting plate 40, blocking completely the fluid communication between the air cooling chamber 50 and the interior of the refrigerating compartment R by the upper part of said compartment. The user can also displace the obturator 60 to other positions which allow the partial or total alignment of its orifices 60a in relation to the corresponding windows 40a of the deflecting plate 40, thus allowing the total or partial liberation of the air inlet upper opening 51 in the air cooling chamber 50. In the construction illustrated in figure 6, the upper opening 51 and the lower opening 52 of the air cooling chamber 50 are defined by a gap which is formed, respectively, between the upper edge 41 of the deflecting plate 40 and an adjacent portion of the dividing wall 11 of the cabinet 10 and between the lower edge 42 of the deflecting plate 40 and the portion of the rear wall 14 of the refrigerating compartment R. In this embodiment shown in figure 6, the obturator 60 is mounted in the gap formed between the lower edge 42 of the deflecting plate 40 and the portion of the rear wall 14 of the refrigerating compartment R. In either of the embodiments illustrated for the upper and lower positioning of the obturator 60, it should be noted that when the obturator is closed, it is no more possible to occur circulation or passage of the descending airflow through the interior of the air cooling chamber 50. In case the obturator is adjusted to allow only a reduced passage of air, the user can limit the airflow through the air cooling chamber 50 and thus adjust the refrigeration capacity of the evaporating plate portion 30 to the refrigeration requirements of the refrigerating compartment R. It should be further understood that when the obturator 60 is closed, the cooling of the refrigerating compartment R is made by heat being transferred from the air to the deflecting plate 40 and therefrom to the ambient of the air cooling chamber 50 refrigerated by the evaporating plate portion 30.