fix the onnx export custom model example; fix repo name; fix opset version; remove deprecated arg;

[janak2]

What does this PR do?

Fix the example for custom model export

Fixes # (issue)

Before submitting

• [ x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ x] Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@fxmarty, @echarlaix, @JingyaHuang, @michaelbenayoun

[echarlaix]

Thanks for the fix @janak2

[HuggingFaceDocBuilderDev]

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