Example code in documentation does not work

[elsni]

On this documentation Page
https://marmelab.com/react-admin/CreateEdit.html#customizing-the-form-layout
The Example code in subsection "formClassName" does not work: makeStyles() is undefined.

[fzaninotto]

I pushed the fix to master by mistake. I've reverted 3355f1d in c32471d, and I'm reopening this issue to propose the fix in a PR instead.