The ornamental design for a stair tread, as shown and described.
FIG. 1 is a perspective view of a stair tread showing my new design as viewed from a front side of the stair tread;
FIG. 2 is a left-side view of the stair tread shown in FIG. 1;
FIG. 3 is a right-side view of the stair tread shown in FIG. 1;
FIG. 4 is a front view of the stair tread shown in FIG. 1;
FIG. 5 is a rear view of the stair tread shown in FIG. 1;
FIG. 6 is a top view of the stair tread shown in FIG. 1; and,
FIG. 7 is a bottom view of the stair tread shown in FIG. 1.
The stair tread is shown with symbolic breaks in its length and width. The appearance of any portion of the article between the break lines forms no part of the claimed design.