The ornamental design for a polisher, as shown and described.
FIG. 1 is a left/front perspective view of a polisher showing my new design.
FIG. 2 is a front view thereof.
FIG. 3 is a rear view thereof.
FIG. 4 is a right side view thereof.
FIG. 5 is a left side view thereof.
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line showing of details in FIGS. 1, 3-4, and 6-7 is included for the purpose of showing environmental portions of the polisher and forms no part of the claimed design.