Fix typo in `Dataset` and `IterableDataset` docs (#28960)
Summary:
Replaced "overrite" with "overwrite".
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28960
Differential Revision: D18246411
Pulled By: soumith
fbshipit-source-id: dc0979a44b7c621a316823061760e0358c227727
