Link to Pandas task doesn't work on GitHub

[hroncok]

It uses xref and that works on https://courses.fit.cvut.cz/MI-PYT but here it links to https://github.com/cvut/MI-PYT/blob/master/_tasks/pandas.adoc instead of https://github.com/cvut/MI-PYT/blob/master/_tasks/pandas.md

I guess a fix is to convert it to AsciiDoc and don't mix markups.