The ornamental design for a headband, as shown and described.
FIG. 1 is a perspective view of a headband showing the new design in use;
FIG. 2 is a perspective view of the headband shown in use, in alternative configuration;
FIG. 3 is a front, side perspective view of the headband;
FIG. 4 is a front view of the headband.
FIG. 5 is a side view of the headband; and,
FIG. 6 is a front, side perspective view of the headband.
The broken line showing of the person in FIGS. 1 and 2 is for the purpose of depicting environmental structure and forms no part of the claimed design.