Fix name of ad slot sizes on gallery pages

Refactored the names of the ad slot sizes, this page slipped through

## Before

![screen shot 2014-09-05 at 12 06 32](https://cloud.githubusercontent.com/assets/74132/4163886/d63291b8-34ec-11e4-8a80-5ddf6c697687.png)

## After

![screen shot 2014-09-05 at 12 06 29](https://cloud.githubusercontent.com/assets/74132/4163888/da0aec5e-34ec-11e4-87bd-85ec4cc08cd4.png)