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>