Corrected link to Device Art Generator (fixes 404 issue)

Goto http://romannurik.github.io/AndroidAssetStudio/

Click the link "Official Android Device Art Generator"

Get's a 404 going to:
http://developer.android.com/distribute/promote/device-art.html

needs to go to:

http://developer.android.com/distribute/tools/promote/device-art.html