Fix: example code of Request-Based Constraints

Author: htetwunsan

guides/source/routing.md

@@ -822,10 +822,8 @@ Will match an incoming request with a path to `admin` subdomain.
 You can also specify constraints by using a [`constraints`][] block:
 
 ```ruby
-namespace :admin do
-  constraints subdomain: "admin" do
-    resources :photos
-  end
+constraints subdomain: "admin" do
+  resources :photos
 end
 ```