The ornamental design for a displaybox, as shown and described.
FIG. 1 is a perspective view of a display box according to the present invention;
FIG. 2 is a left side elevation view thereof; the right side is a mirror image thereto;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The lightweight dashed broken line portion of the figure drawings is included to show unclaimed subject matter only and forms no part of the claimed design; and, the special dot-dash broken line defines the bounds of the claimed design and forms no part of the claimed design.