Documentation/admin-guide: cgroup-v1/rdma: drop doubled word

Drop the doubled word "echo".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: cgroups@vger.kernel.org
Link: https://lore.kernel.org/r/20200704032020.21923-3-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: cgroup-v1: add it to the admin-guide book

Those files belong to the admin guide, so add them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>