Fauxton: Fix placeholder for Username

The one for Password has no `:` so removing them makes the view
more consistent (and looks maybe better)

![bildschirmfoto 2014-03-05 um 22 20 39](https://f.cloud.github.com/assets/298166/2338672/14b25c7e-a4ac-11e3-93d3-6dac32bac33e.png)