The ornamental design for a towel, as shown and described.
FIG. 1 is a front side view of a towel showing my new design;
FIG. 2 is a back side view thereof;
FIG. 3 is a perspective view thereof; and,
FIG. 4 is a reference view showing the towel in use.
The broken line showing of a head and upper torso in FIG. 4 is included for the purpose of showing environmental portions of the towel and forms no part of the claimed design.
The vertical line in FIG. 2 represents a seam and the horizontal lines represent conventional stitching.