The ornamental design for a cap for writinginstrument, as shown and described.
FIG. 1 is a front view of a cap for writing instrument according to the present invention, the rear side view being a mirror image of the front side view;
FIG. 2 is a left side view of the cap for writing instrument of FIG. 1,
FIG. 3 is a right side view of the cap for writing instrument of FIG. 1;
FIG. 4 is a top plan view of the cap for writing instrument of FIG. 1; and,
FIG. 5 is a bottom plan view of the cap for writing instrument of FIG. 1.