The ornamental design for a dentalinstrument, as shown and described.
FIG. 1 is a perspective view of a dental instrument showing the new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left-side view thereof;
FIG. 5 is a right-side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line shown in the figures is for the purpose of illustrating portions of the dental instrument and forms no part of the claimed design. In addition, the surface shading illustrated in the drawings is provided merely to highlight the contour of design; it is not intended to be illustrative of texture or gloss.