The ornamental design for a shelf, as shown and described.
FIG. 1 is a front perspective of a shelf showing our new design;
FIG. 2 is a rear perspective view of the shelf thereof;
FIG. 3 is front view of the shelf thereof;
FIG. 4 is back view of the shelf thereof;
FIG. 5 is side view of the shelf thereof;
FIG. 6 is side view of the shelf thereof;
FIG. 7 is top view of the shelf thereof; and,
FIG. 8 is bottom view of the shelf thereof.
The broken lines represent unclaimed boundaries of the design. The unshaded regions and regions demarcated by broken lines form no part of the claimed design. The broken lines showing the/a string are for environmental purposes only and form no part of the claimed design.