The ornamental design for a writinginstrument, as shown and described.
FIG. 1 is a perspective view of a writing instrument according to the present design;
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;
FIG. 5 is a left side 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 top view of the writing instrument of FIG. 1.
The claimed solid lines are shown on an entirely unclaimed surface. The broken lines are included for the purpose of illustrating parts of the writing instrument that form no part of the claimed design.