Fix typo in ChainDataset docs (#60336)
Summary:
* chainning -> chaining
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60336
Reviewed By: bdhirsh
Differential Revision: D29265236
Pulled By: anjali411
fbshipit-source-id: 17a9b73af9e094550bd1ee25bc9439fb8d455e2b
