Fix a typo in the preprocessing tutorial
# What does this PR do?

Fixed a typo in transformers [tutorials > preprocess](https://huggingface.co/docs/transformers/preprocessing). Currently the code and its output does not match. Please see last two cells from this [colab notebook](https://colab.research.google.com/drive/18WjgFPtQu4n8k6qAWtpsjVwDmICDrcfD#scrollTo=4LsMdS8plf-K); `dataset[0]["image"]` is a PIL image, not a dictionary as present in the current version of the tutorial. It should be `dataset[0]`.

(Please ignore the change at line 490, Github is showing the wrong diff. There's no actual changes.)

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
My fault, there's nothing wrong with the tutorial. 😮‍💨<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19645). All of your documentation changes will be reflected on that endpoint.