The ornamental design for a plug, as shown and described.
FIG. 1 is an upper rear perspective view of the plug;
FIG. 2 is a lower rear perspective view of the plug;
FIG. 3 is another lower rear perspective view of the plug;
FIG. 4 is a lower frontal perspective view of the plug;
FIG. 5 is a front elevational view of the plug;
FIG. 6 is a left side elevational view of the plug;
FIG. 7 is a right side elevational view of the plug;
FIG. 8 is a rear elevational view of the plug;
FIG. 9 is a top plan view of the plug; and,
FIG. 10 is a bottom plan view of the plug.
The long-dash broken lines are for the purpose of illustrating the boundaries of the claimed design. The short-dash broken lines are for the purpose of illustrating environmental subject matter. None of the broken lines form part of the claimed design.