The ornamental design for a wall covering, as shown and described.
FIG. 1 is a front view of a new design for a wall covering;
FIG. 2 is a back view thereof;
FIG. 3 is a bottom view thereof; the top view being a mirror image of FIG. 3;
FIG. 4 is a right side view thereof; the left side view being a mirror image of FIG. 4; and,
FIG. 5 is a perspective view thereof, shown in a position of use.
The broken line perimeters surrounding the design in FIGS. 1-2 and 5 depict boundaries of the design and form no part thereof. The broken lines extending across the side and back surfaces in FIGS. 3 and 4 depict boundaries of the design and form no part thereof. The door and door frame drawn in broken line within a plane on the left hand side of the design are environmental subject matter that forms no part of the claimed design. All broken lines in the drawings are not claimed.