The ornamental design for a wall covering, as shown and described.
FIG. 1 is a top plan view of a wall covering showing my new design.
The undisclosed rear and sides of the wall covering are plain and
unornamented. The drawing further shows only a portion of the wall
covering upon which the design, repeated in units bounded by the broken
lines, is applied. The broken lines are in no way intended to form a part
of the design as shown and are for illustrative purposes only.