Documentation/kmemcheck: update kmemcheck documentation

Kmemcheck configuration menu location correction in Documentation/
kmemcheck.txt

Signed-off-by: Libin <huawei.libin@huawei.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>