docs(usage): update to use json_theme_plus (#1343)

USAGE.md

@@ -1,10 +1,10 @@
 # Using the generated theme
 
-Appainter uses the package [json_theme](https://pub.dev/packages/json_theme) to encode and decode the theme JSON file.
+Appainter uses the package [json_theme_plus](https://pub.dev/packages/json_theme_plus) (credits to the original package [json_theme](https://pub.dev/packages/json_theme)) to encode and decode the theme JSON file.
 
 ## Theme compatibility
 
-From version to version, Flutter may deprecate certain properties in [ThemeData](https://api.flutter.dev/flutter/material/ThemeData-class.html). When this happens, `json_theme` will also update their JSON schema with the latest properties. 
+From version to version, Flutter may deprecate certain properties in [ThemeData](https://api.flutter.dev/flutter/material/ThemeData-class.html). When this happens, the serialisation package will also update their JSON schema with the latest properties.
 
 The Flutter version that Appainter is running will pop-up whenever you use Appainter, on both web and desktop apps. It is highly recommended to use the same Flutter version when running your app. Otherwise, there may be compatibility issues when using the theme JSON file in your app.
 
@@ -14,11 +14,11 @@ If you need to use the theme JSON file in a specific Flutter version, you can fi
 
 To use the theme JSON file in your app, follow the steps below:
 
-1. Add `json_theme` as a dependency in your `pubspec.yaml` file.
+1. Add `json_theme_plus` as a dependency in your `pubspec.yaml` file.
 
     ```yml
     dependencies:
-      json_theme: ^6.4.1
+      json_theme_plus: ^6.6.1
     ```
 
 2. Copy the generated `json` file to your app project and place it under the `assets/` folder.