The ornamental design for a baby sling, as shown and described.
FIG. 1 is a perspective view of a baby sling showing my new design including the infant holding part formed of both low elastic and high elastic knitted fabric;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof; and,
FIG. 4 is a reference view showing the baby sling in use.
The broken line portions of the figure drawings are included to show unclaimed subject matter only and form no part of the claimed design.