bhyve.8: Fix formatting of -G's "w" prefix

"w" is a command modifier, not an argument. Use Cm.

Fixes: 2cdff9918e79 byhve: add option to specify IP address for gdb
MFC after: 3 days

(cherry picked from commit 7abddbb3cdeeff0d4d991aee52e92382e847e8a2)