Revert "DOC(BF): fix assumption about chunks indexes in tutorial_classifiers"

This reverts commit f1f0c9faa5fba9ae9f2449fb6f2a0802510e7890.
BF: Make chunks zero-based (even from OpenFMRI)
BF: Fix remaining technical issues with the tutorial

A conceptual overhaul that fully embraces the OpenFMRI approach is
still pending...