The ornamental design for an arm sling, as shown and described.
FIG. 1 is a front view of the arm sling with the pockets closed;
FIG. 2 is an alternate front view of the arm sling with the pockets open;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof; and,
FIG. 5 is an alternate top view thereof.
The broken lines in the drawings are for the purpose of illustrating portions of the arm sling and form no part of the claim. The broken lines on the arm sling illustrating stitching that forms no part of the claim.