The ornamental design for a keg, as shown and described.
FIG. 1 is a top perspective view of a keg, showing our new design;
FIG. 2 is a front elevational view thereof, the rear elevational view being identical;
FIG. 3 is a left side elevational view thereof, the right side elevational view being identical;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The broken lines in FIGS. 1-4 showing a portion of the top of the keg and two fittings are for the purpose of illustrating portions of the keg that form no part of the claimed design. The keg is shown with a symbolic break in the middle. The appearance of any portion of the article between the break lines forms no part of the claimed design.