doc: update deprecated var usage in documentation examples (#7246)

* test(http): fix HTTPS protocol test by using local HTTPS server instead of external request

* docs: update var usage in documentation examples

* docs: updated var to const

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

README.md

@@ -1104,7 +1104,7 @@ The server will handle it as:
 If your backend body-parser (like `body-parser` of `express.js`) supports nested objects decoding, you will get the same object on the server-side automatically
 
 ```js
-  var app = express();
+  const app = express();
 
   app.use(bodyParser.urlencoded({ extended: true })); // support encoded bodies