The ornamental design for a shower shelf, as shown and described.
FIG. 1 is a perspective view of a shower shelf showing our new design;
FIG. 2 is another perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines immediately adjacent to the shaded areas in the drawings represent the boundaries of the claimed design. The broken lines in the drawings depict environmental subject matter that form no part of the claimed design. None of the broken lines form any part of the claimed design.