Fix improper .Fx macro usage.

Differential Revision:	https://reviews.freebsd.org/D1158
Reviewed by:	wblock@
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274720 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f