docs(tutorial/step-6): add alt attribute to images

See http://www.ssbbartgroup.com/blog/accessible-images-using-angular/

Closes #12569

Author: Adrian Roselli

docs/content/tutorial/step_06.ngdoc

@@ -43,7 +43,7 @@ __`app/index.html`:__
 ...
         <ul class="phones">
           <li ng-repeat="phone in phones | filter:query | orderBy:orderProp" class="thumbnail">
-            <a href="#/phones/{{phone.id}}" class="thumb"><img ng-src="{{phone.imageUrl}}"></a>
+            <a href="#/phones/{{phone.id}}" class="thumb"><img ng-src="{{phone.imageUrl}}" alt="{{phone.name}}"></a>
             <a href="#/phones/{{phone.id}}">{{phone.name}}</a>
             <p>{{phone.snippet}}</p>
           </li>