xml2: use URLs from Wayback Machine

[zmwangx]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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According to http://ofb.net:

9/6/2016: OFB.net suffered a disk failure. We are in the process of restoring basic functionality.

Unfortunately I can't find this elsewhere on the Internet.
#6408.

[dunn]

Thanks! I'll check it off.