docs: fix wording in React Native reusable component description

[hassanahmedk]

Description of what this PR is changing or adding, and why:

Fixes wording in the "How do I create reusable components?" section of the
React Native developers guide. The text previously referred to defining
a class component, but the provided example uses a function component.
This PR updates the description to correctly mention function components
so that the text matches the code example.

Issues fixed by this PR (if any):

N/A (doc clarity fix)

PRs or commits this PR depends on (if any):

None

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[sfshaza2]

lgtm. Thanks for clarifying!

[sfshaza2]

/gcbrun

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 5259fcc):

https://flutter-docs-prod--pr12421-fix-react-native-reusable-c-m7um3bgz.web.app