Fix generation docstring
# What does this PR do?

As mentioned in this [comment](https://github.com/huggingface/transformers/issues/14206#issuecomment-955607275), the `add_prefix_space` argument is not available in `GPT2TokenizerFast.__call__` but `GPT2Tokenizer.__call__`. 

Therefore, this PR fixes this error by switching the fast tokenizer to the slow one.


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@patrickvonplaten, @LysandreJik
Thank you!