[Tokenizer] Change default value of return_dict to True in doc string for apply_chat_template

[kashif]

What does this PR do?

This pull request makes a minor update to the apply_chat_template function in src/transformers/tokenization_utils_base.py, changing the default value of the return_dict parameter from False to True in the docstring to match the default value.

Fixes # (issue)

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https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=43223&sha=344e90

[Cyrilvallez]

Thanks!