The ornamental design for a hearinginstrument, as shown and described.
FIG. 1 is a front view of a hearing instrument showing our new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a top perspective view thereof.
The broken lines in the FIG. 2 view are included for the purpose of illustrating a portion of the hearing instrument that forms no part of the claimed design.