The ornamental design for bowl cover, as shown and described.
FIG. 1 is a front and top perspective view;
FIG. 2 is a back and top perspective view;
FIG. 3 is a front elevation view, and the back, left side, and right side elevation views are identical to the front elevation view of FIG. 3;
FIG. 4 is a top view; and,
FIG. 5 is a bottom view.
The broken line showing of the bottom of the bowl cover is for illustrative purpose only and forms no part of the claimed design. The illustrated bowl cover is rotationally symmetrical.