The ornamental design for a set-top box, as shown and described.
FIG. 1 is a perspective view of our design for a set-top box;
FIG. 2 is a front view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a left side view thereof.
The broken phantom lines depict unclaimed subject matter only and form no part of the claimed design. The bottom view forms no part of the claimed design.