fix(docs): broken testing doc link (pmndrs#1742)

readme.md

@@ -512,7 +512,7 @@ A more complete TypeScript guide is [here](docs/guides/typescript.md).
 - You may wonder how to organize your code for better maintenance: [Splitting the store into separate slices](./docs/guides/slices-pattern.md).
 - Recommended usage for this unopinionated library: [Flux inspired practice](./docs/guides/flux-inspired-practice.md).
 - [Calling actions outside a React event handler in pre React 18](./docs/guides/event-handler-in-pre-react-18.md).
-- [Testing](./docs/guides/testing.mdx)
+- [Testing](./docs/guides/testing.md)
 
 ## Third-Party Libraries