Corrected docblock @return type for AbstractUploadBuilder::getInstance.

The @return type self doesn't respect late binding. So analysers will thing the return type is an AbstractUploadBuilder, which is incorrect. Setting the return type to static will fix this.
@mtdowling the class retuned by that method is neither $this or an instance of the class. It's the $this of another class. I've looked at the other decorators, which all return the current instance ($this) in these cases when overwriting. It's breaking the interface when returning something else.