change "submission" to "set of submissions" in Player Tiers box on Tavern page

Change "submission" to "set of submissions" in the "Player Tiers" box on Tavern page, as requested in [private staff/mods G+ post](https://plus.google.com/u/0/114447825469587375506/posts/YH5UKKGuxbJ).


![screen shot 2014-09-06 at 4 03 54 pm](https://cloud.githubusercontent.com/assets/1495809/4174409/446cc118-358c-11e4-9cec-22471eb98350.png)