The ornamental design for a cap for a bottle, as shown and described.
FIG. 1 is an upper perspective view of the cap for a bottle showing my design.
FIG. 2 is a front elevation, back elevation view, a side elevation view, and an opposite side elevation view thereof, being identical;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a bottom plan view thereof.
The broken lines are shown for the purpose of illustrating the boundaries of the claim as well as threaded components, none of which form part of the claim.