The ornamental design for a cap, as shown and described.
FIG. 1 is a top view showing my new design for a cap;
FIG. 2 is a front elevational view thereof; and,
FIG. 3 is a rear elevational view thereof, wherein the pattern of sinusoidally undulating ridges starting at the left side of FIG. 3 repeats through five periods, and thus results in five equidistant maxima, as the cap is circumferentially rotated 360 degrees.