Fix code block in .md for ipynb (#874)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/874

ipynb uses different requirements for spacing than html/sphinx

This updates it to render properly

Reviewed By: tarun292, angelayi

Differential Revision: D50240561

fbshipit-source-id: a1aa6c1101b58b6239e1efda78ae34a61bc27ed1