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 front view of the writing instrument of FIG. 1;
FIG. 3 is a left side 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 right side view of the writing instrument of FIG. 1;
FIG. 6 is a top plan view of the writing instrument of FIG. 1; and,
FIG. 7 is a bottom plan view of the writing instrument of FIG. 1.
The broken lines showing portions of the writing instrument are included for the purpose of illustrating environment and form no part of the claimed design.