Bump react-dev-utils from 4.2.1 to 4.2.3 in /example

Bumps [react-dev-utils](https://github.com/facebook/create-react-app/tree/HEAD/packages/react-dev-utils) from 4.2.1 to 4.2.3.
- [Release notes](https://github.com/facebook/create-react-app/releases)
- [Changelog](https://github.com/facebook/create-react-app/blob/master/CHANGELOG-1.x.md)
- [Commits](https://github.com/facebook/create-react-app/commits/HEAD/packages/react-dev-utils)

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Author: dependabot[bot]