docs/cgroup: remove some duplicate words

When I tried to add some new entries to cgroup-v2.rst, I found that
the description of memory.events had some repetitive words, so I
tried to delete them.

Signed-off-by: Chunguang Xu <brookxu@tencent.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
1 file changed