The ornamental design for a shelf, as shown and described.
1.1: Front
1.2: Back
1.3: Right
1.4: Left
1.5: Top
1.6: Bottom
1.7: Perspective
1.8: Cross sectional
1.9: Cross sectional
1.10: Cross sectional
1.11: Enlarged
1.12: Enlarged
1.13: Enlarged
1.14: Enlarged
1.15: Enlarged
1.16: Enlarged
1.17: Enlarged
1.18: Enlarged
1.19: Enlarged
1.20: Enlarged
The even spaced broken lines show portions of the shelf that form no part of the claimed design, and the dash-dot broken lines define boundaries of the claimed design and form no part of the claimed design. Fig. 1.8 is a cross-sectional view taken along the A-A line shown in fig. 1.1; Fig. 1.9 is a cross-sectional view taken along the B-B line shown in fig. 1.1; Fig. 1.10 is a cross-sectional view taken along the C-C line shown in fig. 1.1; Fig. 1.11. is an enlarged view between D and D shown in fig. 1.4; Fig. 1.12 is an enlarged view between E and E shown in fig. 1.4; Fig. 1.13 is an enlarged view between F and F shown in fig. 1.9; Fig. 1.14 is enlarged view between G and G shown in fig. 1.9; Fig. 1.15 is an enlarged view between H and H shown in fig. 1.8; Fig. 1.16 is an enlarged view between I and I shown in fig. 1.8; Fig. 1.17 is an enlarged view between J and J shown in fig. 1.10; Fig. 1.18 is an enlarged view between K and K shown in fig. 1.10; Fig. 1.19 is an enlarged view of an area L shown in fig. 1.17; and Fig. 1.20 is an enlarged view of an area M shown in fig. 1.18.