The ornamental design for a headband, substantially as shown and described.
FIG. 1 is a front elevation view of a headband;
FIG. 2 is a rear elevation view of the headband;
FIG. 3 is a right elevation view of the headband;
FIG. 4 is a left elevation view of the headband;
FIG. 5 is a top elevation view of the headband;
FIG. 6 is a bottom elevation view of the headband;
FIG. 7 is a perspective view of the headband being worn by a user with the button closure positioned at the back of the user's head;
FIG. 8 is a front perspective view of the headband being worn by a user with the button closure positioned at the side of the user's head; and,
FIG. 9 is a rear perspective view of the headband being worn by a user with the button closure positioned at the back of the user's head.
Dotted lines representing a user of the headband in FIGS. 7-9 represent environmental matter for illustrative purposes only, and form no part of the claimed design.