fixed typo

Typo on line 100 of closures.md. 'Identically' spelled 'identially':

As seen on the website:

![screen shot 2014-05-22 at 10 27 10 am](https://cloud.githubusercontent.com/assets/5958090/3057457/63425c24-e1d6-11e3-85ab-fbf082260f4b.png)