The ornamental design for a sign, as shown and described.
FIG. 1 is a front view of a sign, embodying my new design;
FIG. 2 is a top view thereof, the bottom view being a mirror image thereof; and,
FIG. 3 is a right side view thereof, the left side view being a mirror image thereof.
The sign is flat. The rear of the sign is unornamented and forms no part of the claimed design. The broken lines are included for showing features that form no part of the claim.