The ornamental design for a dentalinstrument, as shown and described.
FIG. 1 is a front view of dental instrument according to a first embodiment, the right-side, left-side and rear views being identical to the front view;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front view of the dental instrument according to a second embodiment, the right-side, left-side and rear views being identical to the front view;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view.
The broken lines depict environmental subject matter only and form no part of the claimed design.