Fix incorrect tuple unpacking in check_cyclic

[minhnhatnoe]

This PR fixes a typo in numericals.py.   To reproduce the issue, use this problem: a b c = triangle a b c; o = circle o a b c; t = on_tline t a o a, on_line t b c; p = midpoint p t a; e = midpoint e t c ? cyclic a p e b

[mirefek]

I have also noticed this bug. I am closing my issue https://github.com/google-deepmind/alphageometry/issues/20 since it is already a pull request (I haven't realized first).