Correct math formatting for lr scheduler (#28467)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28467
Correcting formatting error from #27874. Also making size of parenthesis more natural.
Closes #27874
Test Plan: Imported from OSS
Differential Revision: D18076085
Pulled By: vincentqb
fbshipit-source-id: cb7c52b347d6d11ea4a2d3c94d00a42f849c0a83
