Modified apiURL JavaScript method to use apply instead of call

I was implementing the apiURL helper in a project I'm working on - by the way, the book has been very helpful - and was having problems with multiple arguments, similar to subscriptions in the book. Changing to apply sorted that issue.

```javascript
var URLs = {
    subscriptions: function(userId, id) {
        return "/api/users/'+ userId +'/subscriptions/' + id;
    }
}
```