Update README.md

Author: lizhuoyuan

README.md

@@ -16,7 +16,7 @@
 
 ## Note
 [v3](https://github.com/OpenFlutter/flutter_screenutil/tree/beta) requires `flutter >= 1.19.0`.  
-[v2](https://github.com/OpenFlutter/flutter_screenutil) for current stable version of flutter.
+[v2](https://github.com/OpenFlutter/flutter_screenutil) requires `flutter < 1.19.0`.  
 
 ## Usage: