Method in tutorial is voi_and_variables rather than voiAndVariables

[nickerso]

Reported by @jonrkarr over on opencor/opencor#2611:

The page below of the tutorial discusses a Python method voiAndVariables. The name of this method is voi_and_variables.
https://tutorial-on-cellml-opencor-and-pmr.readthedocs.io/en/latest/opencor_python.html