The ornamental design for a sock, as shown and described.
FIG. 1 is a perspective view of a sock, showing my new design;
FIG. 2 is a left side view;
FIG. 3 is a right side view;
FIG. 4 is a front view;
FIG. 5 is a back view;
FIG. 6 is a top view; and,
FIG. 7 is a bottom view.
The evenly spaced dashed lines depict stitching that does form part of the claimed design. The dash-dot broken lines depict the boundary of the sock, and form no part thereof. The dash-dot-dot broken lines depict portions of the sock and environmental subject matter that forms no part of the claimed design.