The ornamental design for a a pouch for containing a beverage, as shown and described.
FIG. 1 is a front-top and right-side perspective view of the pouch for containing a beverage showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back 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.
The broken line showing portions of the top of a beverage pouch is included for the purpose of illustrating environmental structure and forms no part of the claimed design.