Clarify plugin/example/pubspec.yaml (#23298)

packages/flutter_tools/templates/app/pubspec.yaml.tmpl

 name: {{projectName}}
 description: {{description}}
+{{#withPluginHook}}
+publish_to: 'none'
+{{/withPluginHook}}
+{{^withPluginHook}}
 
 # The following defines the version and build number for your application.
 # A version number is three numbers separated by dots, like 1.2.43
@@ -8,6 +12,7 @@ description: {{description}}
 # build by specifying --build-name and --build-number, respectively.
 # Read more about versioning at semver.org.
 version: 1.0.0+1
+{{/withPluginHook}}
 
 environment:
   sdk: ">=2.0.0-dev.68.0 <3.0.0"