doc: changed order of invocations in https.request() example.

When you call req.end() before you add .on listeners you get an Error that you can't call .on on undefined.

PR-URL: https://github.com/nodejs/node/pull/9614


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>