The ornamental design for an arm sling, as shown and and described.
FIG. 1 is a side elevational view of an arm sling showing my new design;
FIG. 2 is a bottom view thereof;
FIG. 3 is a front end perspective thereof;
FIG. 4 is a back end perspective view thereof; and,
FIG. 5 is a top plan view thereof.
The broken line showing of an arm and sling strap is included for the purpose of illustrating environmental structure and forms no part of the claimed design.