We claim the ornamental design for a shelving unit, as shown and described.
FIG. 1 is a top front perspective view of a shelving unit;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof;
FIG. 8 is a bottom front perspective view thereof; and,
FIG. 9 is a top rear perspective view thereof.
Uniform broken lines show environmental structure that forms no part of the presently claimed design, while the dash-dot-dash phantom lines of symbolic break illustrate that the marked items can vary dimensionally along the interrupted axis within the scope of the presently claimed design, and the portion of the design between those phantom break lines forms no part of the claimed design.