The ornamental design for a shower shelf, as shown and described.
FIG. 1 is a perspective view of a shower shelf showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a back elevation view thereof;
FIG. 4 is a left-side elevation view thereof;
FIG. 5 is a right-side elevation view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof;
FIG. 8 is a second perspective view thereof; and,
FIG. 9 is another perspective view thereof.
The broken lines depicting part of a razor in FIG. 9 are for the purpose of illustrating environmental structure and form no part of the claimed design.