The ornamental design for a sock, as shown and described.
FIG. 1 is a left side perspective view of a sock of the present invention in a position of use, the right side perspective view being a mirror image thereof;
FIG. 2 is a front view of the sock of FIG. 1 in a flat position; and,
FIG. 3 is a rear view of the sock of FIG. 1 in a flat position.
The broken lines shown in the drawings are included for the purpose of illustrating portions of the sock and form no part of the claimed design.