[webview_flutter] Update sample code. (flutter#4727)

Documentation was updated in app-example file but not in `webview_controller.dart`.   
'output' is a positional parameter now, and `JavascriptMessage` changed to `JavaScriptMessage`

Fixes issue flutter#132727

Author: opxdelwin

packages/webview_flutter/webview_flutter/CHANGELOG.md

@@ -1,5 +1,6 @@
-## NEXT
+## 4.2.3
 
+* Fixes the code sample in the dartdocs for `WebViewController.addJavaScriptChannel`.
 * Updates minimum supported SDK version to Flutter 3.7/Dart 2.19.
 
 ## 4.2.2

packages/webview_flutter/webview_flutter/lib/src/webview_controller.dart

@@ -266,8 +266,8 @@ class WebViewController {
   /// ```dart
   /// final WebViewController controller = WebViewController();
   /// controller.addJavaScriptChannel(
-  ///   name: 'Print',
-  ///   onMessageReceived: (JavascriptMessage message) {
+  ///   'Print',
+  ///   onMessageReceived: (JavaScriptMessage message) {
   ///     print(message.message);
   ///   },
   /// );

packages/webview_flutter/webview_flutter/pubspec.yaml

@@ -2,7 +2,7 @@ name: webview_flutter
 description: A Flutter plugin that provides a WebView widget on Android and iOS.
 repository: https://github.com/flutter/packages/tree/main/packages/webview_flutter/webview_flutter
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+webview%22
-version: 4.2.2
+version: 4.2.3
 
 environment:
   sdk: ">=2.19.0 <4.0.0"