The ornamental design for radiator cap, as shown and described.
FIG. 1 is a perspective view of radiator 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 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.
The broken lines in the drawings represent unclaimed environmental subject matter. The unshaded surfaces immediately adjacent to broken lines are unclaimed.