The ornamental design for a shower curtain, as shown and described.
FIG. 1 is a front elevational view of a shower curtain showing our new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a right-side view thereof;
FIG. 4 is an enlarged perspective view of a detail shown in FIG. 3.
FIG. 5 is a front elevational view thereof;
FIG. 6 is a right-side view thereof;
FIG. 7 is a rear elevational view thereof;
FIG. 8 is a top plan view thereof; and,
FIG. 9 is a bottom plan view thereof.
The evenly broken lines on the surface of the claimed design depict stitching. Dot-dash broken lines depict environmental subject matter which form no part of the claimed design.