The ornamental design for an arm sling, as shown and described.
FIG. 1 is a front view of an arm sling in a folded configuration, showing my design;
FIG. 2 is a rear view thereof;
FIG. 3 is a front view of the arm sling in an open configuration;
FIG. 4 is a rear view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a right side view thereof;
FIG. 8 is a top view thereof; and,
FIG. 9 is a perspective view of the arm sling in a cylindrical configuration.
The broken line showing in the Figures shows portions of the arm sling and forms no part of the claimed design. The arm sling is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claim.