docs: refresh CDN URLs and example JSON headers (#7236)

Co-authored-by: Jay <jasonsaayman@gmail.com>

examples/postMultipartFormData/server.js

@@ -6,7 +6,7 @@ export default function (req, res) {
   req.on('end', function () {
     console.log('POST  received');
     res.writeHead(200, {
-      'Content-Type': 'text/json'
+    'Content-Type': 'application/json'
     });
     res.end();
   });