docs(tutorial/step-6): remove unused class="diagram"

Closes angular#5197

docs/content/tutorial/step_06.ngdoc

@@ -64,7 +64,7 @@ We also added phone images next to each record using an image tag with the {@lin
 api/ng.directive:ngSrc ngSrc} directive. That directive prevents the
 browser from treating the angular `{{ expression }}` markup literally, and initiating a request to
 invalid url `http://localhost:8000/app/{{phone.imageUrl}}`, which it would have done if we had only
-specified an attribute binding in a regular `src` attribute (`<img  class="diagram" src="{{phone.imageUrl}}">`).
+specified an attribute binding in a regular `src` attribute (`<img src="{{phone.imageUrl}}">`).
 Using the `ngSrc` directive prevents the browser from making an http request to an invalid location.