Documentation: update ring-buffer-design.txt

Fix typos, grammos, spellos, hyphenation.
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>