autofs: fix typo in Documentation

Link: http://lkml.kernel.org/r/148577164606.9801.12571810310561599401.stgit@pluto.themaw.net


Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.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>