“Is it fair to burden YouTube with finding content on its site that infringes others’ copyright? Putting the burden on the owners of creative works would require every copyright owner, big and small, to patrol the Web continually on an ever-burgeoning number of sites. That’s hardly a workable or equitable solution. Under the law, the obligation is right where it belongs: on the people who derive a benefit from the creative works and are in the position to keep infringement out of their businesses.”