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 bottom 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
intersecting and broken lines, is applied. The intersecting and broken
lines are in no way intended to form a part of the design as shown and are
for illustrative purposes only.