1213: Fixing indentation in train! docstring r=CarloLucibello a=natema
One code block is not correctly displayed in the doc of [Flux.Optimise.train!
](https://fluxml.ai/Flux.jl/stable/training/training/#Flux.Optimise.train!).
Based on the previous code block, I guess it's an indentation problem.
Co-authored-by: natema <natema@users.noreply.github.com>