Lists example variable name mismatch

[fivaldi]

Thanks Ostrava, we're re-using your materials in Olomouc, so here's one little contribution from us - we discovered some issue with variable name mismatch in this little code example:
(https://github.com/PyLadiesCZ/naucse.python.cz/blob/autumn2021-ostrava/lessons/beginners/list/index.md?plain=1#L267-L268)