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

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'

