INSTRUMENT FOR COSMETIC DEVICES The present invention relates to an instrument for use in cosmetic devices, particularly suitable for the cosmetic treatment and care of the hands and feet. It is common knowledge that cosmetic and personal hygiene treatments include caring for the hands and feet, and particularly the cosmetic care and hygiene of the nails on the hands and feet. Said treatment substantially involves treating both the nail and the cuticle that covers the base and sides of the nail, as well as the skin underneath the nail itself. Various devices are used to implement such treatments, such as nail clippers, files for rounding and smoothing the edges of the nail, and sharp instruments for cosmetic treatments needed both on the nail groove and on the epidermis underneath the nail. The cutting devices of known type substantially comprise a cylindrical instrument-holder or handpiece, complete with an area providing a grip and, at one end, means for retaining or locking the corresponding sharp instrument in place. Said locking means enable the instrument to be replaced and include a fixed jaw, integral with the cylindrical handpiece, and a mobile jaw complete with a threaded pin that is coupled to a corresponding threaded hole made in said handpiece. As for the instrument itself, this generally consists of a shaped flat element consisting of a first area with a sharp cutting edge and a second area suitable for being locked between said jaws. The instrument is thus retained exclusively by the friction developed between the smooth surfaces in contact with the instrument and the jaws. After selecting the sharp instrument they require, users insert the instrument manually, placing the second part between the two open jaws, and then mutually rotate the cylindrical handpiece and the jaw, thereby tightening the jaws in order to lock the instrument in place. Such a device advantageously enables the instrument or blade to be changed, thus enabling the user to suitably configure the device according to the treatment to perform on the patient. A first drawback of the instrument described lies in that, while the user is working on the patient, the instrument tends to move due to the force exerted on it by the user. - l - Another drawback, related to the previous one, lies in that users must stop working on the patient and take action manually to rearrange the instrument in the right position. A further drawback lies in that, in re-adjusting the position of the instrument, users are obliged to take action on the first part of the instrument, thereby risking contaminating the cutting edge that comes into contact with the patient. The object of the present invention is to overcome the aforesaid drawbacks. A particular object of the invention is to provide an instrument for cosmetic devices that enables it to be locked more effectively between said locking means. Another object of the invention is to provide an instrument that, once installed on the corresponding device, can withstand greater forces than the known instruments during their use on the patient. A further object is to provide an instrument that is interchangeable and suitable for installation on devices of known type too. Yet another object is to provide an instrument that is quick and easy to replace. A further, not necessarily final object is to provide an instrument that is simple, economical and reliable, and requires no particular maintenance. Said objects have been achieved through the implementation of an instrument for cosmetic devices, advantageously but not exclusively suitable for use in pedicure and/or manicure treatments, wherein, in accordance with the main claim, there is at least a first part with at least a working profile and at least a second part suitable for being engaged in the locking means of said devices, which is characterized in that said at least a second part is provided with means that cooperate with said locking means of said devices, so that said instrument remains firmly attached to said devices. The aforementioned objects will be highlighted in greater detail in the following description of several preferred embodiments of the invention, provided as indicative and non-restrictive examples, with reference to the attached drawings, wherein: - Figure 1 is an axonometric view of a cosmetic device fitted with an instrument carried out according to the present invention; - Figure 2 shows a longitudinal cross section of the device of Figure 1 complete with the related instrument; - Figure 3 shows an axonometric view of the instrument of Figure 1; - Figure 4 shows a front view of the instrument of Figure 1 ; - Figures 5 and 6 respectively show an axonometric view and a front view of another instrument carried out according to the invention; - Figures 7 and 8 respectively show an axonometric view and a side view of another instrument carried out according to the invention; - Figure 9 shows an axonometric view of another instrument carried out according to the invention; - Figure 10 shows an axonometric view of yet another instrument carried out according to the invention. One embodiment of the instrument for cosmetic devices forming the object of the present invention, particularly suitable for pedicure and/or manicure treatments, is illustrated in the figures from 1 to 4, where it is indicated as a whole by the numeral 1. To be more precise, the instrument 1 in Figure 1 is shown installed on a cosmetic device D of known type, consisting of a handpiece in the form of a tubular body or element A complete with an area providing a grip P, and means B for locking the instrument 1 in place. The locking means B consist of a fixed jaw Gl, integral with the handpiece A, and a mobile jaw G2 complete with a threaded pin L, shown in Figure 2, that is coupled with a corresponding threaded hole made in the handpiece A. In the embodiment of the invention illustrated herein, the cutting instrument 1, illustrated in detail in Figures 3 and 4, includes a thin flat element 2 comprising a first part 3, with a working profile that in this example is constituted by a cutting edge 3a, and a second part 4 suited to be engaged in, i.e. for being coupled with, or inserted between the jaws Gl, G2 of the device D. Advantageously, the second part 4 of the instrument 1 is provided with means, indicated as a whole by the numeral 5, suitable for cooperating with the locking means B on the device D, in order to make the instrument 1 integral with the device D. In the particular embodiment illustrated herein, said means 5 comprise elements 5a projecting from a first surface 2a of the flat element 2. In particular, there are two projecting elements 5a that develop in directions 6 substantially at right angles to the surface 2a of the flat element 2 and substantially parallel to each other. Said projecting elements 5a are created by blanking in the flat element 2. Another embodiment of the invention, indicated as a whole by the numeral 100 in Figures 5 and 6, is differentiated from the one previously described in that there are three projecting elements 5b, 5c, 5d. Said projecting elements 5b, 5c, 5d extend from both sides 2a and 2b of the flat element 2 and develop along axes that are substantially parallel to one another and at right angles to the surface 2a of the flat element 2 of the instrument 100. In this particular case, the projecting element 5d extends in the opposite direction to the other two projecting elements 5b and 5c. A further embodiment of the invention, indicated as a whole by the numeral 200 in Figures 7 and 8, is differentiated from the one previously described in that the projecting elements 5e develop along an axis 7 lying substantially obliquely with respect to the plane defined by the surface 2a of the flat element 2 of the instrument 200. Another embodiment of the invention, indicated as a whole by the numeral 300 in Figure 9, is differentiated from the one previously described in that the means 5 are in the form of several through holes 5f made in the flat element 2 of the instrument. The presence of the holes, obtained by blanking, gives rise to slight projections or burrs that increase the friction between the surface of the instrument and the surface of the jaws Gl, G2, thereby increasing the stability of the connection between the instrument and the device. A further embodiment of the invention, indicated as a whole by the numeral 400 in Figure 10, is differentiated from the one previously described in that the means 5 are in the form of a knurled finish 5g. According to further embodiments of the invention, the instrument features parts with a knurled surface finish suitable for increasing the friction between the jaws and the instrument, equivalent to those described above, said parts being positioned in correspondence with the second part 4, that is, on the surfaces involved in creating the coupling between the instrument and the device. After identifying the instrument with the shape best suited to the treatment required for the patient, the user or chiropodist unscrews the mobile jaw G2 and holds the instrument 1, inserting its second part or area 4 between the two jaws Gl, G2. Then the user mutually rotates the handpiece A and the jaw G2, thereby clamping the jaws Gl, G2 in order to lock the instrument 1 in position, as illustrated in Figure 2. In the example illustrated it should be noted that the means 5, and the projecting elements 5a in particular, tend to fit into the corresponding surfaces of the jaws with which they come into contact, thereby considerably improving the stability of the instrument installed. This increases the clamping force of the jaws. It is clear from the above description that the proposed solution enables the established objects to be achieved. In consideration of their characteristics, the proposed device and instrument are particularly useful for chiropodists and beauticians. Although the invention is described with reference to the attached drawings, upon implementation it may undergo modifications that are all included within the inventive concept expressed in the following claims and consequently protected by the present patent.