I claim the ornamental design for a sprocket, as shown and described.
FIG. 1 is a perspective view of a sprocket showing my new design;
FIG. 2 is a front elevational view thereof, the rear being a mirror image;
FIG. 3 is a left side view thereof, the right side being a mirror image;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a partially-enlarged view thereof; and,
FIG. 7 is a front partially-enlarged view of the sprocket.
The even broken lines in the drawings illustrate portions of the sprocket that forms no part of the claimed design. Furthermore, the dash-dot broken lines in FIGS. 6 and 7 are cutaway lines provided to indicate the boundaries for the portion of the sprocket that is enlarged in the drawings; the dash-dot broken lines forms no part of the claimed design.