docs(ngRepeat): update step_02.ngdoc with challenge

docs/content/tutorial/step_02.ngdoc

@@ -250,6 +250,8 @@ browser is limited, which results in your karma tests running extremely slow.
             <tr><th>row number</th></tr>
             <tr ng-repeat="i in [0, 1, 2, 3, 4, 5, 6, 7]"><td>{{i+1}}</td></tr>
           </table>
+
+  Extra points: try and make an 8x8 table using an additional ng-repeat.
 
 * Make the unit test fail by changing `expect(scope.phones.length).toBe(3)` to instead use `toBe(4)`.