Fix and document Zero Shot Image Classification
# What does this PR do?

This PR fixes 3 issues with `zero-shot-image-classification`

* It does not pick a default model, although there is one. This leads to weird error message

```python
pipe = pipeline("zero-shot-image-classification")
>> ValueError: The task defaults can't be correctly selected. You probably meant "translation_XX_to_YY"
```

* The pipeline is not in the list at top of https://huggingface.co/docs/transformers/main_classes/pipelines
* The filter for available models has a typo, although right now this still leads to no models (ok just one).

## 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 read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@Narsil @sgugger 

_The documentation is not available anymore as the PR was closed or merged._