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>

Documentation/admin-guide/cgroup-v2.rst

@@ -1226,7 +1226,7 @@ PAGE_SIZE multiple when read back.
 
 	Note that all fields in this file are hierarchical and the
 	file modified event can be generated due to an event down the
-	hierarchy. For for the local events at the cgroup level see
+	hierarchy. For the local events at the cgroup level see
 	memory.events.local.
 
 	  low