I claim the ornamental design for a shower arm, as shown and described.
FIG. 1 is a front isometric view of a shower arm;
FIG. 2 is a rear isometric view of the shower arm of FIG. 1;
FIG. 3 is a front elevation view of the shower arm of FIG. 1;
FIG. 4 is a rear elevation view of the shower arm of FIG. 1;
FIG. 5 is a top plan view of the shower arm of FIG. 1;
FIG. 6 is a bottom plan view of the shower arm of FIG. 1;
FIG. 7 is a front view of the shower arm of FIG. 1; and,
FIG. 8 is a rear view of the shower arm of FIG. 1.
The broken lines shown in the drawings are only for purposes both of illustrating use and environment and to define the bounds of the claimed design and form no part of the claimed design.