Add key and improve clarity in people.map() example

[LijuanTang94]

This PR improves the beginner-facing example in the "Rendering data from arrays" section.

Changes:

• Adds a key prop to each <li> element when mapping over the people array (to resolve console warning)

This change aligns the code with React best practices and helps new learners avoid common pitfalls when rendering lists.

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[chipit24]

This is done on purpose. Right under that example they even call out the error:

---

Notice the sandbox above displays a console error [...] You’ll learn how to fix this error later on this page. Before we get to that, let’s add some structure to your data.

I think this is a great way to introduce a basic concept to beginners: not overwhelming them at the start with why a key might be needed, and building on the basic concepts first.