The ornamental design for a shelf, as shown and described.
FIG. 1 is a perspective view of a shelf showing my new design;
FIG. 2 is a top plan view thereof, the bottom plan view being identical thereof; and,
FIG. 3 is a front elevation view thereof, the back elevation view, the right side elevation view, and the left side elevation view being a mirror image thereof.
The broken lines shown in FIGS. 1 through 3 illustrate portions of the shelf that form no part of the claimed design.