We claim the ornamental design for a writing instrument, as shown and described.
FIG. 1 is a perspective view of a writing instrument showing our new design;
FIG. 2 is a front view of the same writing instrument;
FIG. 3 is a back view of the same writing instrument;
FIG. 4 is a left side view of the same writing instrument, the right side view being a mirror image of the left side view;
FIG. 5 is a top view of the same writing instrument; and,
FIG. 6 is a bottom view of the same writing instrument.
The broken lines shown in the drawings of FIGS. 1, 2, 3, 4 and 6 illustrate portions of the writing instrument that form no part of the claimed design.