Docs: Warning Example Outdated

### Are you submitting a **bug report** or a **feature request**?

Bug Report

### What is the current behavior?

The **🏁 React Final Form - Warnings** example provided in the docs has not been updated in some time. The version numbers of React, Final Form and React Final Form are very outdated. Updating them to thew newest version shows an error because `mutators` is no longer returned as a FormRenderProp, but is now part of the FormRenderProp `form`.

### What is the expected behavior?

I'd expect the documentation to reflect the current state of the plugin. I've modified the example so it works again and linked it below

### Sandbox Link

https://codesandbox.io/s/react-final-form-warnings-0nimt

### Other Notes

Thanks, Erik, for a great plugin. It's been a joy to use so far :)