docs: fix StrictMode cat scrolling example

[kenkam]

Steps to Reproduce

In the correct example in StrictMode Fixing bugs found by re-running ref callbacks in development section, click on Millie to change cats, then click any number that is not 0.

Expected

Should scroll to relevant <li>.

Actual

Runtime error 😿

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[github-actions]

Size changes

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌

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