I claim the ornamental design for a wall covering, as shown and described.
FIG. 1 is a front view of a wall covering according to the present design;
FIG. 2 is a back view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a perspective view thereof shown in a position of use.
The broken line perimeter surrounding the design in FIGS. 1-7 depicts the boundaries of the claim and forms no part thereof. The broken lines extending perpendicularly across the front and back surfaces in FIGS. 1, 2 and 7 depict the boundaries of the five inner panels of the design and form no part thereof. In FIG. 7, the broken line surrounding the plane on the left of the design depicts the boundary of environmental subject matter that forms no part of the claimed design. The door and door frame drawn in broken line within this plane are environmental subject matter that forms no part of the claimed design. All broken lines in the drawings are not claimed.