The ornamental design for a writinginstrument, as shown and described.
FIG. 1 is a perspective view showing our new design for a writing instrument;
FIG. 2 is a side view of the writing instrument of FIG. 1, the opposite side view being a mirror image thereof;
FIG. 3 is a front view of the writing instrument of FIG. 1;
FIG. 4 is a rear view of the writing instrument of FIG. 1;
FIG. 5 is a top plan view of the writing instrument of FIG. 1; and,
FIG. 6 is a bottom plan view of the writing instrument of FIG. 1.
The broken line showing of a push button and the writing tip and its housing is included for the purpose of illustrating environment and forms no part of the claimed design.