I claim the ornamental design for a plaque, as shown and described.
FIG. 1 is a perspective view of a plaque, showing the new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left-side elevational view thereof, the right-side elevational view being a mirror image thereof;
FIG. 5 is an exploded top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines showing a message and a plurality of small plaques in the figures depict environmental matter, and form no part of the claimed design.