locking, rtmutex.c: Documentation cleanup

Two minor updates on functions documentation:
 - Updated documentation for function rt_mutex_unlock(), which contained an
   incorrect name
 - Removed extra '*' from comment in function rt_mutex_destroy()

[ Impact: cleanup ]

Signed-off-by: Luis Henriques <henrix@sapo.pt>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <20090429205451.GA23154@hades.domain.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>