fix documentation for zero_shot_object_detection
Author: not-lain
Pull request: yes

# What does this PR do?

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Fixes # (issue)
fixes broken documentation for [zero_shot_object_detection](https://huggingface.co/docs/transformers/tasks/zero_shot_object_detection) 



## 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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      Pull Request section?
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- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


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Models:

- text models: @ArthurZucker and @younesbelkada
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@stevhliu and @MKhalusova can help out 

Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_28267). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.