fix($anchorScroll): fix broken example code with correct controller name

The demo does not work
http://docs.angularjs.org/api/ng.$anchorScroll
![screenshot 2013-10-09 13 03 40](https://f.cloud.github.com/assets/1016365/1301180/e97f04dc-311d-11e3-9114-bb2d13a4892c.png)

Rename "MainCtrl" ng-controller to correct controller name "ScrollCtrl"