Content enhancement: lab app mapping

[noraj]

Disclaimer: The course seems to be written with something like a SaaS gitbook and the source is not public so I'll open an issue rather than a PR.

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On the exercise, URL of the app are give but are dead, eg. http://xxe-workshop.gosec.co:8021/ on https://gosecure.github.io/xxe-workshop/#2.

In the introduction it's said to deploy the app with https://github.com/GoSecure/xxe-workshop.

But there is no trivial mapping between http://xxe-workshop.gosec.co:8021 and 21_rssviewer_xxe. One could make the link between Atom feed mentioned in the page and rss viewer or deduce the 21 correspond to the port 80821 but it would be better to mention the app required for this exercise is clearly 21_rssviewer_xxe and maybe with a link to it.