FIG. 1 is a perspective view of a showerhead showing our new design; FIG. 2 is a rear perspective view thereof; FIG. 3 is a front elevational view thereof; FIG. 4 is a rear view thereof; FIG. 5 is a left side view thereof, the opposite side being a mirror image of the side shown; FIG. 6 is a top plan view thereof; and, FIG. 7 is a bottom plan view thereof. The broken line showing included for the purpose of illustrating and forms no part of the claimed design.