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>