Add unsubscribe() to preventing-going-back example

[kulak91]

Hi there! 👋 I've added the unsubscribe() method to the "preventing-going-back" example. This addition ensures that we clean up the event listener when the component unmounts, preventing potential memory leaks and bugs.

The unsubscribe() method is called when the component is unmounted, which removes the listener set up for the beforeRemove event. This is important for maintaining a clean and efficient navigation flow by avoiding unnecessary event listeners stacking after the component is no longer in use.

Feel free to review the changes and let me know if there are any questions or feedback. Thanks! 😊

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