Update readme (do not mention specific version) (#4038)

* Update readme for Next 5

* remove specific version mention

readme.md

@@ -62,7 +62,7 @@ Install it:
 npm install --save next react react-dom
 ```
 
-> Next.js 4 only supports [React 16](https://reactjs.org/blog/2017/09/26/react-v16.0.html).<br/>
+> Next.js only supports [React 16](https://reactjs.org/blog/2017/09/26/react-v16.0.html).<br/>
 > We had to drop React 15 support due to the way React 16 works and how we use it.
 
 and add a script to your package.json like this: