The ornamental design for a “writing instrument”, as shown and described.
FIG. 1 is a front, bottom, and right side perspective view of our new design for a writing instrument.
FIG. 2 is a front view of the writing instrument of FIG. 1.
FIG. 3 is a rear view of the writing instrument of FIG. 1.
FIG. 4 is a right side view of the writing instrument of FIG. 1, the left side view being identical thereto.
FIG. 5 is a top view of the writing instrument of FIG. 1.
FIG. 6 is a bottom view of the writing instrument of FIG. 1; and,
FIG. 7 is a rear view of the writing instrument of FIG. 1 with the writing tip covered.
The portions of the writing instrument shown in broken lines are for illustrative purposes only in FIGS. 1, 2, and 6 and form no part of the claimed design.