Correct AutoConfig call docstrings
Author: Sebelino
Pull request: yes

# What does this PR do?

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The `AutoConfig` class has no method named `from_json_file`, so the [examples in the documentation](https://huggingface.co/transformers/model_doc/auto.html#transformers.TFAutoModelForSequenceClassification.from_pretrained) are incorrect. Most likely the intention is to call `from_pretrained`.

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- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
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