Update README.md

Author: lizhuoyuan

README.md

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 # flutter_screenutil
-[![pub package](https://img.shields.io/pub/v/flutter_screenutil.svg)](https://pub.dartlang.org/packages/flutter_screenutil)
+[![pub package](https://img.shields.io/pub/v/flutter_screenutil.svg)](https://pub.dev/packages/flutter_screenutil)
 
 **A flutter plugin for adapting screen and font size.Let your UI display a reasonable layout on different screen sizes!**