We claim the ornamental design for a sink shelf, as shown and described.
FIG. 1 is a top, front, left perspective view of the claimed design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a rear elevation view thereof;
FIG. 6 is a right elevation view thereof; and,
FIG. 7 is a left elevation view thereof.
The ornamental design which is claimed is shown in solid lines in the drawings. The broken lines in the drawings are for illustrative purposes only and form no part of the claimed design. Broken lines formed by equal length dashes show unclaimed subject matter. Broken lines formed by unequal length dashes (i.e., dash-dot) define bounds of the claimed design. Broken away symbols having a jagged element show that no particular length of the portion is claimed. Portions of the claimed design having line shading and stipple shading define contrasting surfaces.