[Doc] [ONNX]Fix a broken url for ONNXRuntime custom op (#67944)
Summary:
**Description**
Update the broken url by a valid link https://onnxruntime.ai/docs/reference/operators/add-custom-op.html.
**Motivation**
Closes https://github.com/pytorch/pytorch/issues/67849. The url is broken.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67944
Reviewed By: NivekT
Differential Revision: D32252880
Pulled By: H-Huang
fbshipit-source-id: 400b0efa3d6f63e60b016c482fbbed1293c29806
