autofs: rename autofs documentation files

There are two files in Documentation/filsystems that should now use
autofs rather than autofs4 in their names.

Link: http://lkml.kernel.org/r/152626707957.28589.3325300375892913999.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>