The ornamental design for a sock, as shown and described.
FIG. 1 is a front, left perspective view of a sock showing our new design;
FIG. 2 is a front, right perspective view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a front view thereof; and,
FIG. 6 is a rear view thereof.
The uneven-length broken lines immediately adjacent to the soiid lines define the bounds of the claimed design. The even-length broken lines showing the remainder of the sock represent environmental subject matter. None of the broken lines form part of the claimed design.