Update example test documentation

Example does not work with session headers, should use request headers. [ci skip]

actionpack/lib/action_controller/metal/http_authentication.rb

@@ -53,10 +53,8 @@ module HttpAuthentication
     # In your integration tests, you can do something like this:
     #
     #   def test_access_granted_from_xml
-    #     get(
-    #       "/notes/1.xml", nil,
-    #       'HTTP_AUTHORIZATION' => ActionController::HttpAuthentication::Basic.encode_credentials(users(:dhh).name, users(:dhh).password)
-    #     )
+    #     @request.env['HTTP_AUTHORIZATION'] => ActionController::HttpAuthentication::Basic.encode_credentials(users(:dhh).name, users(:dhh).password)
+    #     get "/notes/1.xml"
     #
     #     assert_equal 200, status
     #   end