Gen 2 flutter add auth sample code needs rename outputs file

[hangoocn]

Describe the content issue: The sample code is:

import 'amplifyconfiguration.dart';
await Amplify.configure(outputs);

but these should be renamed to match the actual output generated:

import 'amplify_outputs.dart';
await Amplify.configure(amplifyConfig);

URL page where content issue is: https://docs.amplify.aws/flutter/start/quickstart/#adding-authentication