update print instructions in the documentation

[nbehrnd]

By the previous commmit, the documentation still included the old Python2 print statement. It has been superseeded by Python3's print function this commit adopts. On occasion, the results querring the PubChem database differ from the earlier version of the documentation which are considered, too.

A check run of the functionality described by the documentation (except for code in advanced.rst, not tested) with Python 3.9.2, pandas 1.3.1, jupyter-notebook 6.2.0, PubChemPy 1.0.4 in Linux Debian 11/bullseye, branch testing, was successful.