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>

doc/api/https.md

@@ -158,11 +158,11 @@ var req = https.request(options, (res) => {
     process.stdout.write(d);
   });
 });
-req.end();
 
 req.on('error', (e) => {
   console.error(e);
 });
+req.end();
 ```
 
 The options argument has the following options