Update implementation-notes.md

[hardfist]

prevChild could be CompositeComponent instance which cause prevChild.node to be undefined.

Before submitting a pull request, please make sure the following is done:

1. Fork the repository and create your branch from master.
2. If you've added code that should be tested, add tests!
3. If you've changed APIs, update the documentation.
4. Ensure the test suite passes (npm test).
5. Format your code with prettier (npm run prettier).
6. Make sure your code lints (npm run lint).
7. Run the Flow typechecks (npm run flow).
8. If you haven't already, complete the CLA.

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[bvaughn]

Hi @hardfist! Thanks so much for contributing to the React documentation.

You may not have been aware of issue #11075 but we were in the process of moving documentation to its own repository. It is now live at a new location: https://github.com/reactjs/reactjs.org

Please re-open this PR there (if you're willing). We apologize for the inconvenience!

Cheers!