wave1D_fd2: 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/02_wave/wave1D_fd2.ipynb`, or [here](https://nbviewer.jupyter.org/github/devitocodes/devito_book/blob/master/fdm-devito-notebooks/02_wave/wave1D_fd2.ipynb).

Linked with #27 
