The ornamental design for display box, as shown and described.
FIG. 1 is a perspective view of display box showing our new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is another perspective view in use.
The broken lines showing of the wheels in the drawing are for the purpose of illustrating environmental structure and form no part of the claimed design.