Update "custom components with fallback" tip page

[jakeboone02]

https://github.com/react-querybuilder/react-querybuilder.github.io/edit/main/docs/tips/custom-with-fallback.mdx

This page is based on the assumption that option groups are not supported, but they will be once react-querybuilder/react-querybuilder#265 is merged/released. Come up with a different example.