Critical Bugfix: Run nbdev_clean to make sure nbs/index.ipynb conform to the standard

[deephbz]

The bug is a big blocker but easy to fix. I explain everything in the PR: AnswerDotAI/nbdev-template#19

[deephbz]

@jph00 @hamelsmu when you get a chance please take a look at the above PR. It's an easy one liner change unblocking new users