The ornamental design for a plug, as shown and described.
FIG. 1 is a top, front, right side perspective view of a plug showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The dashed broken lines of the figures represent portions of the plug that form no part of the claimed design. The dash-dot broken lines define boundaries of the claimed design, and form no part of the claimed design.