fix SyncBatchNorm doc (#20991)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/19265
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20991
Differential Revision: D15513518
Pulled By: soumith
fbshipit-source-id: 9618c0b2442e013e4d37793cdb04cb4f4b1b141c
