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: #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>

doc/api/process.markdown

@@ -340,7 +340,7 @@ An example of the possible output looks like:
          target_arch: 'x64',
          v8_use_snapshot: 'true' } }
 
-### process.connected
+## process.connected
 
 * {Boolean} Set to false after `process.disconnect()` is called