I claim the ornamental design for a door threshold, as shown and described.
FIG. 1 is a front isometric view of the door threshold;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a side view of the door threshold in use, wherein no claim is made to the matter represented by dashed lines which forms environmental structure; and,
FIG. 7 is an alternate side view of the door threshold in use.