The ornamental design for a sticker, as shown and described.
FIG. 1 is a perspective view of a sticker in accordance with one embodiment of the present invention; and
FIG. 2 is a front view of the sticker of FIG. 1; and,
FIG. 3 is a perspective view of a sticker in accordance with another embodiment of the present invention.
The sticker is intended for attachment to a protective helmet. The embodiments of the stickers shown in FIGS. 1 and 3 are mirror images of each other. The rear or back view of the sticker is not part of the claimed design.