docs: correct 'undefined' to 'null' in callback example

The example comment shows `undefined` but logs `null` due to sync execution.

Signed-off-by: Benjamin <127757412+pirateIV@users.noreply.github.com>

Author: pirateIV

apps/site/pages/en/learn/asynchronous-work/event-loop-timers-and-nexttick.md

@@ -347,7 +347,7 @@ function someAsyncApiCall(callback) {
 // the callback is called before `someAsyncApiCall` completes.
 someAsyncApiCall(() => {
   // since someAsyncApiCall hasn't completed, bar hasn't been assigned any value
-  console.log('bar', bar); // undefined
+  console.log('bar', bar); // null
 });
 
 bar = 1;