Input: Documentation/sysrq.txt - update KEY_SYSRQ info

While setting up sysrq operation on the XO laptop (which lacks a SysRq
key), i realized that the documentation was quite out of date.

Change documentation of SysRq to reflect current KEY_SYSRQ value.

Signed-off-by: Paul Fox <pgf@laptop.org>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>