Fix broken link (#16965)
Fix broken link (#16965)

See https://docs.flutter.io/ to see what I mean.
flutter/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter