sysrq docs: document sequence that actually works

Document sequence of keypresses that actually works. Yes, this changed
year-or-so ago.

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>