The ornamental design for a cap of pen, as shown and described.
FIG. 1 is a perspective view of the top, front and left side of a cap of pen showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof; and,
FIG. 7 is a cross-sectional view thereof at 7—7 in FIG. 3.
The broken lines showing inside portions of the cap of pen in FIG. 6 are included for the purpose of illustrating environment and form no part of the claimed design.