case sensitive path for ipynb/Jotto.ipynb and ipynb/jotto.ipynb

[bmerkle]

there is a problem when checking out on windows because Jotto.ipynb is also used in lower cases only.

Problem:
git clone ....

warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

'ipynb/Jotto.ipynb'
'ipynb/jotto.ipynb'

[bmerkle]

Hello Peter @norvig
can you merge this PR, because on windows this results in problems when users to a simple git clone
The good thing is that the fix is trivial :-)
thanks