The ornamental design for a plaque, as shown and described.
FIG. 1 is a perspective view of a plaque;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof, the right side view being a mirror image thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines showing in the figures are included for the purpose of illustrating environment of the design and form no part of the claimed invention.