We claim the ornamental design for a plug assembly, as shown and described.
FIG. 1 is a front perspective view of a plug assembly;
FIG. 2 is a back perspective view of the plug assembly;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a left elevational view thereof;
FIG. 6 is a right elevational view thereof;
FIG. 7 is a back plan view thereof; and,
FIG. 8 is a front plan view thereof.
The broken lines shown in the drawings of the plug assembly are for the purpose of illustrating portions of the article and form no part of the claimed design.