bhyve.8: Fix a typo

The Li macro is deprecated. Also, the Cm macro should be used here
instead for consistency with the rest of the manual and style.mdoc(5).

Fixes: e47fe3183e1f bhyve: add ROM emulation
MFC after: 1 month

(cherry picked from commit 5e19a5185324dedc1fb245e1d7871148ef39a155)