The ornamental design for a coffee sling, as shown and described.
FIG. 1 is a perspective view of a coffee sling in accordance with the present invention;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevation thereof;
FIG. 4 is a side elevation thereof; and,
FIG. 5 is a bottom plan view thereof.
In FIGS. 1-5, the hatched lines depict character and contours of the surfaces and the broken lines depict environmental subject matter only and form no part of the claimed design.