FIG. 1 is a front right perspective view of an ornamental design for a writing instrument according to the present invention;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a back elevational view thereof;

FIG. 4 is a left side elevational view thereof;

FIG. 5 is a right side elevational view thereof;

FIG. 6 is a bottom plan view thereof; and,

FIG. 7 is a top plan view thereof.

All portions shown in broken lines are environmental and illustrative only and form no part of the claimed design.