Fix doc unwanted dl Admin:: [ci skip]

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -717,7 +717,7 @@ def map_method(method, args, &block)
       #   resources :posts, module: "admin"
       #
       # If you want to route /admin/posts to +PostsController+
-      # (without the Admin:: module prefix), you could use
+      # (without the <tt>Admin::</tt> module prefix), you could use
       #
       #   scope "/admin" do
       #     resources :posts