The ornamental design for a set-top box, as shown and described.
FIG. 1 is a front side perspective view illustrating a set-top box according to our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a back side perspective view thereof.
The broken lines depict portions of the set top box that form no part of the claimed design. The broken lines and the unshaded regions bounded by broken lines illustrate boundaries and, therefore, form no part of the claimed design. Non-oblique line shading is meant to show contour and not surface ornamentation.