Variable `psi` is not used in "data_extract.ipynb"

In the cell 'Calculating Rotation Matrix for vehicle', the variable `psi` is not used, if it is not necessary maybe that line could be removed. 

However if it should be used in the equations below, maybe there has been a typo (phi/psi)?

It is quite important to know because if the case is the latter, then rotation results would be wrong.

Thanks.