Clarify dynamic import stage in docs (#7813)

Author: aprilandjan

docusaurus/docs/supported-browsers-features.md

@@ -15,7 +15,7 @@ This project supports a superset of the latest JavaScript standard. In addition
 - [Exponentiation Operator](https://github.com/rwaldron/exponentiation-operator) (ES2016).
 - [Async/await](https://github.com/tc39/ecmascript-asyncawait) (ES2017).
 - [Object Rest/Spread Properties](https://github.com/tc39/proposal-object-rest-spread) (ES2018).
-- [Dynamic import()](https://github.com/tc39/proposal-dynamic-import) (stage 3 proposal)
+- [Dynamic import()](https://github.com/tc39/proposal-dynamic-import) (stage 4 proposal)
 - [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).