The ornamental design of a cap for a bottle, as shown and described.
FIG. 1 is a perspective view of a cap for a bottle showing our new design.
FIG. 2 is a front view thereof.
FIG. 3 is a rear view thereof.
FIG. 4 is a right side view thereof.
FIG. 5 is a left side view thereof.
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
In the drawings, the dashed broken lines show portions of a cap for a bottle that form no part of the claim. The dot-dash lines form a boundary of the claimed design and form no part thereof.