The ornamental design for a plug, as shown and described.
FIG. 1 is a perspective view of the plug;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is a cross-sectional view at cross-section 8-8 of FIG. 5; and,
FIG. 9 is a sectional view thereof, but shown with environment in broken lines.
The environment and portions of the plug shown in broken lines form no part of the claimed design. The long dash-short dash line forms the boundary of the claimed design and forms no part of the claimed design.