autofs: use autofs instead of autofs4 in documentation

Finally remove autofs4 references in the filesystems documentation.

Link: http://lkml.kernel.org/r/152626709055.28589.416082809460051475.stgit@pluto.themaw.net
Signed-off-by: Ian Kent <raven@themaw.net>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>