cgroups: Documentation/cgroup/cgroup.txt - a trivial fix.

This trivial patch removes a word which appears twice in
Documentation/cgroup/cgroup.txt.
Signed-off-by: NRami Rosen <ramirose@gmail.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

Documentation/cgroups/cgroups.txt

@@ -442,7 +442,7 @@ You can attach the current shell task by echoing 0:
 You can use the cgroup.procs file instead of the tasks file to move all
 threads in a threadgroup at once. Echoing the PID of any task in a
 threadgroup to cgroup.procs causes all tasks in that threadgroup to be
-be attached to the cgroup. Writing 0 to cgroup.procs moves all tasks
+attached to the cgroup. Writing 0 to cgroup.procs moves all tasks
 in the writing task's threadgroup.
 
 Note: Since every task is always a member of exactly one cgroup in each