docs(guide/expression): fix duplicate key error in example
docs(guide/expression): fix duplicate key error in example

The example that demonstrates how to parse expressions can fail if you
pass in the same expression twice.  By using "track by $index" we can
fix this.

Closes #4472

ammula88/angular.js,ammula88/angular.js,ammula88/angular.js,ammula88/angular.js,ammula88/angular.js