docs($rootScope): code example for $watch function using objectEquality

This PR improves the code example section of https://docs.angularjs.org/api/ng/type/$rootScope.Scope#$watch by providing an example of `scope.$watch` with `objectEquality` to illustrate how `scope.$watch` will not work simply as a reference watcher on object properties. 

Note: first time contributor, CLA may be pending.