fix(part4b/README): correct broken link to callback hell explanation Replaced the invalid Stack Overflow link with a relevant and working answer: https://stackoverflow.com/a/25098230

Author: ABfathy

src/content/4/en/part4b.md

@@ -397,7 +397,7 @@ Note.find({}).then(notes => {
 
 The _Note.find()_ method returns a promise and we can access the result of the operation by registering a callback function with the _then_ method.
 
-All of the code we want to execute once the operation finishes is written in the callback function. If we wanted to make several asynchronous function calls in sequence, the situation would soon become painful. The asynchronous calls would have to be made in the callback. This would likely lead to complicated code and could potentially give birth to a so-called [callback hell](http://callbackhell.com/).
+All of the code we want to execute once the operation finishes is written in the callback function. If we wanted to make several asynchronous function calls in sequence, the situation would soon become painful. The asynchronous calls would have to be made in the callback. This would likely lead to complicated code and could potentially give birth to a so-called [callback hell](https://stackoverflow.com/a/25098230).
 
 By [chaining promises](https://javascript.info/promise-chaining) we could keep the situation somewhat under control, and avoid callback hell by creating a fairly clean chain of _then_ method calls. We have seen a few of these during the course. To illustrate this, you can view an artificial example of a function that fetches all notes and then deletes the first one: