The ornamental design for a headband, as shown and described.
FIG. 1 is a perspective view of a headband showing my new design;
FIG. 2 is a front elevation view of the headband;
FIG. 3 is a side elevation view of the headband, with the opposite side being a mirror image thereof;
FIG. 4 is a top plan view of the headband;
FIG. 5 is a bottom plan view of the headband;
FIG. 6 is a rear elevation view of the headband;
FIG. 7 is a side view of the headband, shown in an environment of use; and,
FIG. 8 is a back view of the headband, shown in an environment of use.
The broken line showing of the woman in FIGS. 7 and 8 depicts environment and forms no part of the claimed design. All other broken lines depict portions of the headband that form no part of the claimed design.