Updated copy on section error to include student enrollment example.

@rocha @clintonb @lamagnifica 

![screen shot 2014-10-01 at 11 23 23 am](https://cloud.githubusercontent.com/assets/5265058/4477557/f7be1b30-497e-11e4-97c1-0bf878436820.png)

I just made the change--didn't see your message until after the commit.  The one advantage that hasn't been talked about is if we manage to backfill the data, we can change the date in the setting rather than in code.

I don't feel strongly either way.  The setting seems strong for something so simple and that won't change often and hardcoding it seems wrong if the date were to change.  I guess I see more advantages for the setting.