docs(tutorial/step-4): "unknown" option is actually blank

The name 'unknown' doesn't appear as a choice, the new choice is just blank.
Side note: once I choose one of the non-blank options, I no longer see the blank option.

Closes angular#10079

Author: bwestrich

docs/content/tutorial/step_04.ngdoc

@@ -182,7 +182,7 @@ You can now rerun `npm run protractor` to see the tests run.
 # Experiments
 
 * In the `PhoneListCtrl` controller, remove the statement that sets the `orderProp` value and
-you'll see that Angular will temporarily add a new "unknown" option to the drop-down list and the
+you'll see that Angular will temporarily add a new blank ("unknown") option to the drop-down list and the
 ordering will default to unordered/natural order.
 
 * Add an `{{orderProp}}` binding into the `index.html` template to display its current value as