I'm pretty sure that's just an XML namespace name (or URI). Even though it looks like a URL, the parser just treats it like a string. It's not used to look up or retrieve any information. There MAY be something at that address if you put a URI into a browser, but there certainly doesn't HAVE to be.
If you're looking for the specs for epub 2.0.1, you'll find them at:
http://idpf.org/epub/201