Documentation: add missing angle bracket in cgroup-v2 doc

Trivial addition of missing closing angle backet.

Signed-off-by: Joel Savitz <jsavitz@redhat.com>
Link: https://lore.kernel.org/r/20220422164526.3464306-1-jsavitz@redhat.com


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