docs: cgroup-v1/blkio-controller.rst: remove a CFQ left over

changeset fb5772cb ("blkio-controller.txt: Remove references to CFQ")
removed cgroup references to CFQ, but it kept one left. Get rid of
it.

Fixes: fb5772cb

 ("blkio-controller.txt: Remove references to CFQ")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>