doc: Fix inaccuracy in https.request docs

PR-URL: https://github.com/nodejs/node/pull/9453


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Sam Roberts <sam@strongloop.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>