Replaced render :text with render :plain in AC gem bug report template

- Followup of https://github.com/rails/rails/pull/20929.
 [ci skip]

guides/bug_report_templates/action_controller_gem.rb

@@ -32,7 +32,7 @@ class TestController < ActionController::Base
   include Rails.application.routes.url_helpers
 
   def index
-    render text: 'Home'
+    render plain: 'Home'
   end
 end