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

lib/libc/gen/posix_spawn.3

@@ -300,7 +300,9 @@ had been called to create a child process and then
 .Fn execve
 had been called by the child process to execute the new process image.
 .Pp
-The implementation uses vfork(), thus the fork handlers are not run when
+The implementation uses
+.Fn vfork ,
+thus the fork handlers are not run when
 .Fn posix_spawn
 or
 .Fn posix_spawnp