fix: add type definitions and fix arrow function param

[tomaszwysocki]

I modified the User type to include the appropriate values. Additionally, I enclosed a sole arrow function parameter in parentheses, following the same convention enforced by Nest's Prettier.

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[micalevisk]

I'm not sure if we should make this example more complete and complicated (to TS newbies) tho

[tomaszwysocki]

I believe this is a rather straightforward example, and verifying that utility type shouldn't pose much difficulty. In my opinion, it aligns well with Nest's methodology of avoiding longer explanations, encouraging readers to explore on their own.