Replace 'React' with 'react' in rntester examples (#37185)

Summary:
React Native macOS:
Please note, I do not plan to

Pull Request resolved: https://github.com/facebook/react-native/pull/37185

Updates a few imports in RNTester to 'react' as we had previously done in:
https://github.com/facebook/react-native/commit/0ee5f689

Changelog: [Internal]

Reviewed By: jacdebug

Differential Revision: D45445211

fbshipit-source-id: 7d345a874bd8b99ddb11ebb5385c2a07330a6531