Use relative URI to xml.xsd

Hi, thank you for this pull request. I’m sorry that it’s taken so long to get back to you about it. Because of the length of time that’s passed since O’Reilly has actively reviewed pull requests to this project from outside contributors, and because of the context lost in that time, we’re closing all stale pull requests.

If you still feel we should consider this pull request, please feel free to reopen, and we’ll review it in a timely manner. Thanks again!