[docs] fix formula `torch.logcumsumexp` (#38952)
Summary:
Reference : https://github.com/pytorch/pytorch/pull/36308#issuecomment-632282641
After fix:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38952
Differential Revision: D21722196
Pulled By: ezyang
fbshipit-source-id: 62b08c14e0ce9603133841940627df40d7b1e861
