update link for difference between proposal stages (#6149)

seems the page which the old link points to has removed the related information.

Author: loveky

docusaurus/docs/supported-browsers-features.md

@@ -19,7 +19,7 @@ This project supports a superset of the latest JavaScript standard. In addition
 - [Class Fields and Static Properties](https://github.com/tc39/proposal-class-public-fields) (part of stage 3 proposal).
 - [JSX](https://facebook.github.io/react/docs/introducing-jsx.html), [Flow](./adding-flow) and [TypeScript](./adding-typescript).
 
-Learn more about [different proposal stages](https://babeljs.io/docs/plugins/#presets-stage-x-experimental-presets-).
+Learn more about [different proposal stages](https://tc39.github.io/process-document/).
 
 While we recommend using experimental proposals with some caution, Facebook heavily uses these features in the product code, so we intend to provide [codemods](https://medium.com/@cpojer/effective-javascript-codemods-5a6686bb46fb) if any of these proposals change in the future.