The ornamental design for a boot, as shown and described.
FIG. 1 is a front perspective view of the boot showing my new design.
FIG. 2 is a front view thereof.
FIG. 3 is a back view thereof.
FIG. 4 is a side view thereof.
FIG. 5 is another side view thereof; and,
FIG. 6 is a top view thereof.
The dashed lines on the boot upper represents claimed stitching. The dashed lines forming the sole and the inner boot surface as seen in FIG. 6 represent unclaimed subject matter and form no part of the claim.