Fix outdated link for how to edit plugin code (#18069)

* Fix readme link to plugin edit instructions

* Fix flutter tools link

packages/flutter_tools/lib/src/commands/create.dart

@@ -206,7 +206,7 @@ Your main program file is lib/main.dart in the $relativeAppPath directory.
 Your plugin code is in lib/$projectName.dart in the $relativePluginPath directory.
 
 Host platform code is in the android/ and ios/ directories under $relativePluginPath.
-To edit platform code in an IDE see https://flutter.io/platform-plugins/#edit-code.
+To edit platform code in an IDE see https://flutter.io/developing-packages/#edit-plugin-package.
 ''');
         }
       } else {

packages/flutter_tools/templates/plugin/README.md.tmpl

@@ -7,4 +7,4 @@
 For help getting started with Flutter, view our online
 [documentation](https://flutter.io/).
 
-For help on editing plugin code, view the [documentation](https://flutter.io/platform-plugins/#edit-code).
\ No newline at end of file
+For help on editing plugin code, view the [documentation](https://flutter.io/developing-packages/#edit-plugin-package).