docs: sched-bwc.rst: fix a typo on a doc name

cgroupv2.rst -> cgroup-v2.rst

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1dc0203bd7df375ef45832f0c88566e22c4138ff.1621413933.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>