The ornamental design for a sock, as shown and described.
FIG. 1 is a first side perspective view of a sock showing my new design.
FIG. 2 is a second side perspective view thereof;
FIG. 3 is a first side view thereof;
FIG. 4 is a second side view thereof; and,
FIG. 5 is a bottom view of the sock in a laid-flat configuration showing my new design.
The long and short broken lines depict a boundary of the claim and form no part of the claim. The even-length broken lines depict environment and form no part of the claim.