Shelf

FIG. 1 is a perspective view of a shelf showing the new design for a shelf; FIG. 2 is a front view thereof; FIG. 3 is a rear view thereof; FIG. 4 is a right view thereof; FIG. 5 is a left view thereof; FIG. 6 is a top view thereof; and, FIG. 7 is a bottom view thereof. The broken lines show portions of the shelf that form no part of the claimed design.