Minor error fix of 'bart-large-cnn' details in the pretrained_models doc
I found that there seemed to be a mistake regarding "facebook/bart-large-cnn" in the pretrained_models doc.

# What does this PR do?

While I check the model explanations in the pretrained_models doc, I found that there seemed to be a mistake.

Regarding `facebook/bart-large-cnn`, the details of the model is as follows:

```
12-layer, 1024-hidden, 16-heads, 406M parameters (same as base)
bart-large base architecture finetuned on cnn summarization task
```

If my understanding is correct, it seems that `12-layer` and `(same as base)` should be `24-layer` and `(same as large)`.

I asked a question in the forum about this:
https://discuss.huggingface.co/t/there-seems-to-be-a-mistake-in-documentation-pretrained-models-html-regarding-bart/1746/

## 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?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to the 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?

documentation: @sgugger
(Thank you for kindly answering my question in the forum!)

Thanks!<|||||>Thank you, too!