The ornamental design for an armband, as shown and described.
FIG. 1 is an isometric view of an armband that has the design.
FIG. 2 is a front view of the armband seen in FIG. 1.
FIG. 3 is a back view of the armband seen in FIG. 1.
FIG. 4 is a right-side view of the armband seen in FIG. 1.
FIG. 5 is a left-side view of the armband seen in FIG. 1.
FIG. 6 is a top view of the armband seen in FIG. 1; and,
FIG. 7 is a bottom view of the armband seen in FIG. 1.
The broken lines around the central face in the isometric view and in the front view and around the flap in the bottom view are intended to represent stitching and form part of the claimed design. The other broken lines represent unclaimed portions of the armband and form no part of the claimed design.