doc: fix formatting in process.markdown

All the other properties get an h2/## but process.connected gets an
h3/### for no discernible reason. Change it to h2/## like the others.

PR-URL: https://github.com/nodejs/node/pull/4433


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Stephan Belanger <admin@stephenbelanger.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>