Documentation: sysrq, description of 'h' slightly inaccurate

In Documentation/sysrq.txt, the description of 'h' says that any key not
listed *above* will generate help.  That's obviously not true since all the
keys listed below 'h' will do what they are described to do, not display help.
 So change the text so that it says that any key not listed in the table will
generate help, which is what really happens.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>