Correct passive form for departments on the main edition filter

After discussion with folk in the content team, the verdict was that the possessive form for things ending in "s" should not have the extra "s".

Before:

![screen shot 2014-10-29 at 12 44 15](https://cloud.githubusercontent.com/assets/3687/4825819/6de83872-5f69-11e4-9078-c968898189f5.png)

After:

![screen shot 2014-10-29 at 12 43 41](https://cloud.githubusercontent.com/assets/3687/4825821/757913ea-5f69-11e4-9809-d4942f156390.png)