Update model-hook.md (emberjs#2269)

Test fails if     
assert.equal(this.element.querySelectorAll('.results .listing').length, 3, 'should display 3 listings');
because listings are not in an element with class "results"

source/tutorial/model-hook.md

@@ -124,7 +124,7 @@ import {
 ```/tests/acceptance/list-rentals-test.js{+2,+3}
 test('should list available rentals.', async function(assert) {
  await visit('/');
- assert.equal(this.element.querySelectorAll('.results .listing').length, 3, 'should display 3 listings');
+ assert.equal(this.element.querySelectorAll('.listing').length, 3, 'should display 3 listings');
 });
 ```