The ornamental design for an 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 back elevation view of the headband;
FIG. 4 is a side elevation view of the headband;
FIG. 5 is a top plan view of the headband;
FIG. 6 is a bottom plan 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 of the headband depict portions and features of the headband that form no part of the claimed design. The broken line outline of the partial human figure in FIGS. 7 and 8 assists in illustrating how the headband may appear when worn, but forms no part of the claimed design.