docs(choicefield): update docs in demo
docs(choicefield): update docs in demo

Added note about using `ng-value` for `non-string` `ngModel`.

References #462.

ngOfficeUIFabric/ng-officeuifabric,jjczopek/ng-officeuifabric,andrewconnell/ng-officeuifabric,andrewconnell/ng-officeuifabric,ngOfficeUIFabric/ng-officeuifabric,ngOfficeUIFabric/ng-officeuifabric,jjczopek/ng-officeuifabric,ngOfficeUIFabric/ng-officeuifabric,andrewconnell/ng-officeuifabric,ngOfficeUIFabric/ng-officeuifabric,jjczopek/ng-officeuifabric,andrewconnell/ng-officeuifabric,jjczopek/ng-officeuifabric,andrewconnell/ng-officeuifabric,jjczopek/ng-officeuifabric