I claim the ornamental design for a boot jack, as shown.
FIG. 1 is a top plan view of the boot jack showing my new design.
FIG. 2 is a bottom plan view of the boot jack.
FIG. 3 is a right side view of the boot jack (the left side view begin a mirror image of the right side view).
FIG. 4 is a rear view of the boot jack; and,
FIG. 5 is a front view of the boot jack.
The broken line showing of details in FIGS. 1, 2, 4, and 5 is included for the purpose of showing boundaries and the environmental portions within the boundaries of the boot jack and forms no part of the claimed design.