The ornamental design for a showerhead, as shown and described.
FIG. 1 is a perspective view of a showerhead showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a left side view thereof, the opposite side being a mirror image of the side shown;
FIG. 4 is a cross-sectional view thereof taken along line 4-4 in FIG. 1;
FIG. 5 is a top end view thereof;
FIG. 6 is a bottom end view thereof; and,
FIG. 7 is a rear view thereof.
The broken line showing of the front water nozzles and rear attachment is included for the purpose of illustrating and forms no part of the claimed design.