The ornamental design for a button for a window covering, as shown and described.
FIG. 1 is a top perspective view of a button for a window covering showing our design in a position of use;
FIG. 2 is a front view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof; and,
FIG. 5 is a left side view thereof, the right side view being a mirror image thereof.
The rear of the button cannot be seen when the button is in use. The broken lines in FIGS. 1-5 represent environmental structure that forms no part of the claimed design.