The ornamental design for a boot, as shown and described.
FIG. 1 is a left side view of a boot showing my boot design;
FIG. 2 is a right side view of the boot shown in FIG. 1; and,
FIG. 3 is a front view of the boot shown in FIG. 1.
The broken line showing of the bottom portion of the boot, top rim of the boot, zipper, and zipper track, is for illustrative purposes only and forms no part of the claimed design.