The ornamental design for a headband, as shown and described.
FIG. 1 is a perspective view of the headband;
FIG. 2 is a front elevation view of the headband;
FIG. 3 is a side elevation view of the headband;
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 in use; and,
FIG. 8 is a back view of the headband in use.
The broken lines along the headband depict features of the headband that form no part of the claimed design. The partial human figure outlined by broken lines in FIGS. 7 and 8 assists to illustrate how the headband may appear when in use and forms no part of the claimed design.