Document formatting (#42065)
Summary:
Apply syntax highlighting to the command in `README.md`. This makes `README.md` easier to read.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42065
Reviewed By: pbelevich
Differential Revision: D22753418
Pulled By: mrshenli
fbshipit-source-id: ebfa90fdf60478c34bc8a7284d163e0254cfbe3b
