The ornamental design for a cap for a beveragecontainer, as shown and described.
FIG. 1 is a perspective view of the cap for a beverage container of the present disclosure;
FIG. 2 is a first side elevational view of the cap for a beverage container;
FIG. 3 is a second side elevational view of the cap for a beverage container;
FIG. 4 is a top plan view of the cap for a beverage container;
FIG. 5 is a front elevational view of the cap for a beverage container;
FIG. 6 is a rear elevational view of the cap for a beverage container;
FIG. 7 is a bottom plan view of the cap for a beverage container; and,
FIG. 8 is a side elevational view of the cap for a beverage container shown on a beverage container.
The dot dot dash lines depict portions of the cap for a beverage container. The dash dash lines depict the boundaries of the claim. The broken lines form no part of the claimed design.