The ornamental design for a cap for a bottle, as shown and described.
FIG. 1 is a front view of a cap for a bottle showing my new design;
FIG. 2 is a top view thereof; and,
FIG. 3 is a bottom view thereof.
The evenly broken lines are shown for the purpose of describing symbolic break edges and form no part of the claimed design. The dot-dash lines in FIG. 2 represent the boundaries of the claim and form no part thereof; the unshaded area adjacent to each boundary and toward the center of the design is excluded from the claim. The broken away symbols in the drawings indicate that any portion of the article beyond what is shown forms no part of the claimed design.