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.
This is because of how instance resolving is setup. The class doesn't return a new instance of itself, it passes it's namespace to a resolver. In theory this method could be placed in any other class in the same namespace and it would still resolve to the same instance. So, it is indeed the same class. But it otherwise it should be `self` and not static like the others. Because this setup doesn't allow for extension unless you follow the same FQCN resolving and thus structure.