doc: fixup code sample in http.md

PR-URL: #38776
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

doc/api/http.md

@@ -2831,7 +2831,9 @@ class. The `ClientRequest` instance is a writable stream. If one needs to
 upload a file with a POST request, then write to the `ClientRequest` object.
 
 ```js
-const postData = querystring.stringify({
+const http = require('http');
+
+const postData = JSON.stringify({
   'msg': 'Hello World!'
 });
 
@@ -2841,7 +2843,7 @@ const options = {
   path: '/upload',
   method: 'POST',
   headers: {
-    'Content-Type': 'application/x-www-form-urlencoded',
+    'Content-Type': 'application/json',
     'Content-Length': Buffer.byteLength(postData)
   }
 };