The ornamental design for a plug, as shown and described.
FIG. 1 is a top, front, right side perspective view 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. The pairs of bracketed adjacent wavy break lines form no part of the claimed design, and indicate that the appearance of any portion of the plug between the break lines forms no part of the claimed design.