[baseio.py] Correct name of attribute
[baseio.py] Correct name of attribute

Change name of attribute from extentions to extensions.
rgerkin/python-neo,mgr0dzicki/python-neo,JuliaSprenger/python-neo,INM-6/python-neo,NeuralEnsemble/python-neo,apdavison/python-neo,samuelgarcia/python-neo