[DOCS] Quiver should be in `dependencies` but not `dev_dependencies`

[greyovo]

I added quiver to dev_dependencies in pubspec.yaml, according to README, and run

flutter pub run json_to_model

it outputs:

Unhandled exception:
Quiver is needed to generate models (To support `null` in copyWith and mocks). Please run `flutter pub add quiver`

#0      PubspecParser.parse (package:json_to_model/config/pubspec_parser.dart:26:7)
<asynchronous suspension>
#1      main (file:///F:/SDK/flutter_windows_2.10.5-stable/flutter/.pub-cache/hosted/pub.dev/json_to_model-3.1.8/bin/json_to_model.dart:7
:26)
<asynchronous suspension>
pub finished with exit code 255

but in dependencies section it works.

Flutter SDK version: 2.10.5 stable, Android Studio 2021.1.1