doc: python3 instead of python
doc: python3 instead of python

Otherwise, python2 would be used in most systems.

SchrodingersGat/InvenTree,inventree/InvenTree,SchrodingersGat/InvenTree,inventree/InvenTree,SchrodingersGat/InvenTree,inventree/InvenTree,inventree/InvenTree,SchrodingersGat/InvenTree