http: replace var with const on code of comment

`const` or `let` is more preferred than `var` except iteration.

PR-URL: #45951
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: theanarkh <theratliter@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Zeyu "Alex" Yang <himself65@outlook.com>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/_http_server.js

@@ -257,8 +257,8 @@ function onServerResponseClose() {
   // array. That is, in the example below, b still gets called even though
   // it's been removed by a:
   //
-  //   var EventEmitter = require('events');
-  //   var obj = new EventEmitter();
+  //   const EventEmitter = require('events');
+  //   const obj = new EventEmitter();
   //   obj.on('event', a);
   //   obj.on('event', b);
   //   function a() { obj.removeListener('event', b) }