Fix docstring of description about input_ids
Author: nxznm
Pull request: yes

# What does this PR do?

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This PR fixes a docstring of description about input_ids in `DistilBertForSequenceClassification` class.

Fixes #11659 

## Before submitting
- [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).
- [ ] Did you write any new necessary tests?


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@NielsRogge @sgugger 
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