Fix buffer

A small fix in offset computation or some example were not working in:
https://github.com/rougier/loops-tutorial

(Tutorial with Cyrille in Paris)

% formatting is [obsolete](https://docs.python.org/release/3.1.5/library/stdtypes.html#old-string-formatting-operations) in Python 3 (see also [this](http://stackoverflow.com/questions/5082452/python-string-formatting-vs-format/5082482#5082482)). But let's worry about that later if the rest of the codebase uses the old syntax.