We claim the ornamental design for a bottle cap, as shown and described.
FIG. 1 is a top perspective view of a bottle cap, showing our new design.
FIG. 2 is a top plan view of the bottle cap; and,
FIG. 3 is a front plan view of the bottle cap, in which the rear, left side, and right side views are identical.
The bottom view is unornamented and not shown. Phantom lines in the drawings indicate an environment in which the bottle cap includes an grip detail that forms no part of the claimed design.