Fix docstring of CLIPTokenizer(Fast)
# What does this PR do?

The docstrings of `CLIPTokenizer` and `CLIPTokenizerFast` had the wrong default value for `bos_token` (`<|endoftext|>` instead of `<|startoftext|>`).

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?

- Documentation: @sgugger
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20233). All of your documentation changes will be reflected on that endpoint.