We claim the ornamental design for a dental instrument, as shown and described.
FIG. 1 is a perspective view of the dental instrument.
FIG. 2 is a first side view thereof.
FIG. 3 is a second side view thereof.
FIG. 4 is a top plan view thereof.
FIG. 5 is a bottom plan view thereof.
FIG. 6 is a front view thereof; and,
FIG. 7 is a back view thereof.
The broken lines immediately adjacent the shaded area represent the bounds while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.