Typo in css/aloha.css

Fixed typo: "pin.pn" should be "pin.png" in aloha.css

changed "background:url(/Users/rene/evo42/app/aloha/Aloha-Editor/src/img/pin.pn) to background:url(/Users/rene/evo42/app/aloha/Aloha-Editor/src/img/pin.png)
Merge pull request #1 from grobins2/patch-1

Typo in css/aloha.css