autofs: fix typos in Documentation/filesystems/autofs4.txt

plus minor whitespace fixes.

Link: http://lkml.kernel.org/r/20160812024734.12352.17122.stgit@pluto.themaw.net
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>