The ornamental design for a sock, as shown and described.
FIG. 1 is a rear perspective view of a sock showing our new design;
FIG. 2 is a side elevational view, the opposite side being a mirror image thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken away symbols in the drawings indicate the appearance of any portion of the article between the break lines forms no part of the claimed design.