Fix header setting doc in testing guide [ci skip]

guides/source/testing.md

@@ -1113,11 +1113,10 @@ end
 
 Now you can try running all the tests and they should pass.
 
-NOTE: If you followed the steps in the Basic Authentication section, you'll need to add the following to the `setup` block to get all the tests passing:
+NOTE: If you followed the steps in the Basic Authentication section, you'll need to add authorization to every request header to get all the tests passing:
 
 ```ruby
-request.headers['Authorization'] = ActionController::HttpAuthentication::Basic.
-  encode_credentials('dhh', 'secret')
+post articles_url, params: { article: { body: 'Rails is awesome!', title: 'Hello Rails' } }, headers: { Authorization: ActionController::HttpAuthentication::Basic.encode_credentials('dhh', 'secret') }
 ```
 
 ### Available Request Types for Functional Tests