The ornamental design for a writinginstrument, as shown and described.
FIG. 1 is a front elevation view of a writing instrument showing the new design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is an enlarged left side elevation view thereof;
FIG. 6 is an enlarged right side elevation view thereof; and,
FIG. 7 is a rear, left side, and top perspective view thereof.
The broken lines showing the top, middle band, bottom and clip in FIGS. 1-7 are included for the purpose of showing portions of the writing instrument that form no part of the claimed design.