doc: Update DCO to v1.1

PR-URL: https://github.com/nodejs/node/pull/6353


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Mikeal Rogers <mikeal.rogers@gmail.com>