Update README.md

Author: lizhuoyuan

README.md

@@ -15,7 +15,7 @@
 [Update log](https://github.com/OpenFlutter/flutter_screenutil/blob/master/CHANGELOG.md)
 
 ## Note
-[v3](https://github.com/OpenFlutter/flutter_screenutil/tree/beta) requires `flutter >= 1.19.0`. 
+[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.
 
 ## Usage: