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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What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature
• [x] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [x] Docs
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• [x] No

[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.