Documentation/dm-init: fix multi device example

The example in the docs regarding multiple device-mappers is invalid (it
has a wrong number of arguments), it's a left over from previous
versions of the patch.
Replace the example with an valid and tested one.

Signed-off-by: Helen Koike <helen.koike@collabora.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>