diffu_fd1: Check and correct formatting

Make sure all formatting is correct. See wiki and [this readme](https://github.com/devitocodes/devito_book/blob/master/fdm-devito-notebooks/README.md) for more info.

**Available at:**  `fdm-devito-notebooks/03_diffu/diffu_fd1.ipynb`, or [here](https://nbviewer.jupyter.org/github/devitocodes/devito_book/blob/master/fdm-devito-notebooks/03_diffu/diffu_fd1.ipynb).

Linked with #37 
