[ci skip] Fix syntax error + make example easier to follow.

Follow up to 9f152a60

actionpack/lib/action_controller/metal/http_authentication.rb

@@ -56,8 +56,9 @@ module HttpAuthentication
     # In your integration tests, you can do something like this:
     #
     #   def test_access_granted_from_xml
-    #     headers = { 'HTTP_AUTHORIZATION' => ActionController::HttpAuthentication::Basic.encode_credentials(users(:dhh).name, users(:dhh).password }
-    #     get "/notes/1.xml", headers: headers
+    #     authorization = ActionController::HttpAuthentication::Basic.encode_credentials(users(:dhh).name, users(:dhh).password)
+    #
+    #     get "/notes/1.xml", headers: { 'HTTP_AUTHORIZATION' => authorization }
     #
     #     assert_equal 200, status
     #   end