FIG. 1 is a perspective view showing the front, top, and left sides of our new design for a shower wall. FIG. 2 is a front view of our new design for a shower wall. FIG. 3 is a rear view of our new design for a shower wall. FIG. 4 is a top view of our new design for a shower wall; and, FIG. 5 is a detail view of our new design for a shower wall in the region circled in FIG. 4. Broken lines in the drawing figures represent the environment and do not constitute part of the claimed design.