Fix doc build (#42559)
Summary:
Add space between double back quotes and left curly bracket
Otherwise doc generation failed with `Inline literal start-string without end-string.`
This regression was introduced by https://github.com/pytorch/pytorch/commit/b56db305cf441453a84ae0b55f708ac0998cafc0
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42559
Reviewed By: glaringlee
Differential Revision: D22931527
Pulled By: malfet
fbshipit-source-id: 11c04a92dbba48592505f704d77222cf92a81055
