The ornamental design for a boot, as shown and described.
FIG. 1 is a front perspective view of a boot showing my new design;
FIG. 2 is a right side view thereof; and,
FIG. 3 is a left side view thereof.
The broken line showing of the sole, heel, zipper, zipper track, and portion of the toe is for illustrative purposes only and forms no part of the claimed design. The stitching showing around the lacework and top of the boot is a claimed feature of the design.