The ornamental design for a shower curtainhook, as shown and described.
FIG. 1 is a perspective view of an embodiment of a shower curtain hook according to my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a left side elevation view thereof, the right side being a mirror image thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a back elevation view thereof.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.