The ornamental design for a sticker, as shown and described.
FIG. 1 is a perspective view of the sticker shown in use attached to a vehicle.
FIG. 2 is a front plan view of FIG. 1.
FIG. 3 is a right side elevation view of FIG. 2.
FIG. 4 is a bottom elevation view of FIG. 2.
FIG. 5 is a rear plan view of FIG. 2.
FIG. 6 is a left side elevation view of FIG. 2; and,
FIG. 7 is a top elevation view of FIG. 2.
In the drawings, the broken lines showing of a vehicle illustrates the environment in which the sticker is used and forms no part of the claimed design.