The ornamental design for a head covering, as shown and described.
FIG. 1 is a front perspective view of a head covering showing my new design;
FIG. 2 is a rear perspective view thereof, the broken line showings of a wearer of the head covering being for purposes of illustrating environmental structure forming no part of the claimed design;
FIG. 3 is a right side elevation view thereof, the left side elevation view being a mirror image;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a rear elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown on the head covering itself illustrate stitching, and form a part of the claimed design.