I claim the ornamental design for a spreader cap, as shown.
FIG. 1 is a perspective view of a spreader cap, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof, and the left side is a mirror image of the right side;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines contained in FIG. 6 show the attachment points for component parts of the spreader cap and non-ornamental features of the underside of the cap, are included for the purpose of illustrating environmental structure, and form no part of the claimed design.