bhnd_erom.9: remove self-reference
Change .Xr to .Fn, which quiets a mandoc warning.
Reviewed by: mhorne
MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/1135
(cherry picked from commit 81727a3dd38287d437d1e7e416dd89107a706e55)