[PATCH] typo fixes for rt-mutex-design.txt

Address some simple typos in rt-mutex-design.txt It also changes the
indentation of the cmpxchg example (the cmpxchg example was indented by
spaces, while all other code snippets were indented by tabs).

Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jan Altenberg <tb10alj@tglx.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>