Documentation: fix echo command in cgroups/cpuacct.txt

Must echo a task id to the cgroups' tasks file, but not to a directory.
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/cgroups/cpuacct.txt

@@ -23,7 +23,7 @@ New accounting groups can be created under the parent group /sys/fs/cgroup.
 
 # cd /sys/fs/cgroup
 # mkdir g1
-# echo $$ > g1
+# echo $$ > g1/tasks
 
 The above steps create a new group g1 and move the current shell
 process (bash) into it. CPU time consumed by this bash and its children