The ornamental design for a writinginstrumentcap, as shown and described.
FIG. 1 is a perspective view of an ornamental design for a writing instrument cap according to the present invention;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof rotated 180 degrees from FIG. 6.
The broken lines in FIGS. 1 and 4 to 7 are included for the purpose of showing portions of the writing instrument cap that form no part of the claimed design.