docs: cgroup-v1: fix footnotes

The documentation contains external references, which some of them are
marked as footnotes. Fix the syntax for them to be properly rendered as
such.

Non-footnote references aren't affected since the text for these is
aligned the same to the footnotes.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>