docs: update step_04.ngdoc with a clarification

closes angular#5730

Author: naomiblack

docs/content/tutorial/step_04.ngdoc

@@ -88,8 +88,8 @@ phonecatApp.controller('PhoneListCtrl', function ($scope) {
 record. This property is used to order phones by age.
 
 * We added a line to the controller that sets the default value of `orderProp` to `age`. If we had
-not set the default value here, the model would stay uninitialized until our user picks an
-option from the drop down menu.
+not set a default value here, the `orderBy` filter would remain uninitialized until our 
+user picked an option from the drop down menu.
 
   This is a good time to talk about two-way data-binding. Notice that when the app is loaded in the
 browser, "Newest" is selected in the drop down menu. This is because we set `orderProp` to `'age'`