The ornamental design for a sticker, as shown and described.
FIG. 1 is a front view of a sticker, embodying my new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a left side view thereof; and,
FIG. 4 is a front perspective view thereof, showing the sticker in its environment of use.
The broken lines showing the game controller, in FIG. 4, illustrate the environmental structure of the sticker that forms no part of the claimed design.