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

[PatrickJS]

The demo does not work
http://docs.angularjs.org/api/ng.$anchorScroll

Rename "MainCtrl" ng-controller to correct controller name "ScrollCtrl"

[mary-poppins]

Thanks for the PR!

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Thanks again for your help!

[PatrickJS]

I signed the CLA under the name Patrick Stapleton

[petebacondarwin]

Duplicate of #3898