topdog.tex: broken link captf.com

The link https://captf.com/practice-ctf/ found under Appendix B is broken in the current document.

I found http://54.212.176.14/practice-ctf/ through google but it feels wrong to link to an IP address.

Another option to link to instead would be https://overthewire.org/wargames/ or perhaps https://github.com/apsdehal/awesome-ctf