Fix typos at ONNX docs (#66090)
Summary:
This PR fixes small typos at ONNX docs
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66090
Reviewed By: albanD
Differential Revision: D31385765
Pulled By: ezyang
fbshipit-source-id: f4879069a2acf9c8adaa81c26a6a5014634761f5
