Bumped required React version in documentation (reduxjs#1238)

Author: hedgerh

website/versioned_docs/version-7.x/introduction/quick-start.md

@@ -12,7 +12,7 @@ original_id: quick-start
 
 ## Installation
 
-React Redux 6.x requires **React 16.4 or later.**
+React Redux 7.x requires **React 16.8.4 or later.**
 
 To use React Redux with your React app: