Added mathjax reference on algorithm-archivists/plugin-mathjax

[PudottaPommin]

This is applicable if

• Updated to cdnjs provider plugin-mathjax#1 is merged
• Check locally if dependency on updated plugin is correctly download in gitbook install

I set version which is currently as fallback 2.6.1 but it's possible use any version, just changing book.json for future :)

[leios]

Alright, the PR has been merged. What do you mean with the second box, "Reverified correct updating from Github repository"?

[PudottaPommin]

It was just someone else roo test locally if the plugin is correctly downloading from master branch after PR in his repo.

[june128]

I tested it yesterday and it worked fine, so I'll go ahead and approve this one. Thank you very much @MichalHanajik !