Removed comma

Location: In one of the earlier sections of the book under Collections and 'Adding and Removing Models'

Change suggested: Did not edit anything besides removing what I thought was an unnecessary comma. It seems there shouldn't be a comma when there is only 1 item inside an an .extend({})


BEFORE
![screen shot 2014-11-14 at 7 18 44 am](https://cloud.githubusercontent.com/assets/8462738/5047073/d9fe6fa6-6bce-11e4-8149-e31898f8658b.png)






AFTER
![screen shot 2014-11-14 at 7 21 48 am](https://cloud.githubusercontent.com/assets/8462738/5047090/094896a6-6bcf-11e4-8ab1-cf2ce3390b5d.png)