[Doc]fix torch.ceil formula issue(pytorch#54948) (#55039)
Summary:
Fixes wrong formula https://github.com/pytorch/pytorch/issues/54948
The new one is
<img width="157" alt="截屏2021-03-31 下午5 25 59" src="https://user-images.githubusercontent.com/32546978/113124411-14407000-9248-11eb-92f6-7b47b4cfd5e4.png">
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55039
Reviewed By: ngimel
Differential Revision: D27562484
Pulled By: mruberry
fbshipit-source-id: e01d9bfc0cf04558ecff3336a055037e6c3df028
