Documentation: Update path to sysrq.txt

Commit 9d85025b

 ("docs-rst: create an user's manual book") moved the
sysrq.txt leaving old paths in the kernel docs.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>