posix_spawn(3): fix section that references vfork

vfork(2) should be referenced in paragraphs as .Fn vfork, not vfork().
This change switches the reference to use .Fn, which in turn makes the
manpage make manlint clean.

MFC after: 1 week
Reported by: make manlint
Sponsored by: DellEMC Isilon

(cherry picked from commit 2c96ea9ca0aa37db109700be5ed33afc30133ec1)