memcg: fix typos in memcg_test.txt

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Reviewed-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>