I claim the ornamental design for a adjustable headband, as shown and described.
FIG. 1 is a perspective view of an adjustable headband in accordance with the present design;
FIG. 2 is a right view thereof;
FIG. 3 is a left view thereof;
FIG. 4 is a bottom view thereof, showing by dashed line a portion 5;
FIG. 5 is a portion of the bottom view thereof from FIG. 4;
FIG. 6 is a top view thereof;
FIG. 7 is a front view thereof, showing by dashed line a portion 8;
FIG. 8 is a portion of the front view thereof from FIG. 7;
FIG. 9 is a back view thereof; and,
FIG. 10 is a front view thereof showing the adjustable headband in a different position.
The broken line showings of the adjustable headband are included for the purpose of illustrating portions of the crystal award and form no part of the claimed design.