Fix rendering of equations in dumux-course

Bug report

What happened / Problem description:
Within the dumux-course gitLab pages, some of the Latex equations are not rendered properly, see for example this page.

What you expected to happen:
The math environment should not be depicted but the rendered equations.

How to reproduce it (as minimally and precisely as possible):
Open the gitLab pages in a browser (for me it was Firefox)