The ornamental design for a shower curtain, as shown and described.
FIG. 1 is a right-perspective view of a shower curtain, shown in a stowed configuration in a position of use with shower stall;
FIG. 2 is a front view thereof, shown in an unrolled position;
FIG. 3 is a rear view thereof;
FIG. 4 is a right-side view thereof;
FIG. 5 is a left-side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a right-perspective view thereof, shown in a position of use with a shower stall.
The long dashed broken lines depicting small circular regions on the shower curtain represent portions of the shower curtain that form no part of the claim. The broken lines depicting a rod, rod endcaps, and faucet illustrates environmental subject matter that forms no part of the claim.