update _retina-image.scss to use correct prefix

as it specifically says in Apples documentation, you shouldn't use the @2x prefix.

>A common file-naming convention for high-resolution images on the web is to prepend _2x >before the file extension; for example, myImage_2x.jpg. Don’t use @2x in the filename, >since the @ character in a URL is reserved as the delimiter separating the username and >password from the host in authentication URL schemes.

[reference](http://developer.apple.com/library/safari/#documentation/NetworkingInternet/Conceptual/SafariImageDeliveryBestPractices/ServingImagestoRetinaDisplays/ServingImagestoRetinaDisplays.html#//apple_ref/doc/uid/TP40012449-CH3-SW1)