Clarify extra_special_tokens documentation to match the example
Author: samyuktahegde
Pull request: yes

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# What does this PR do?

This documentation mentions saving the tokenizer and accessing it later from 'output_dir', but the following example only demonstrates the use of 'extra_special_tokens' keyword argument. This PR updates the explanation to match the given example.

If the original intention was to demonstrate saving and reloading the tokenizer, I'd be happy to update the example as per the original explanation.

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Fixes # (issue)

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### CI recap

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Hi! I noticed this PR was closed. I'd appreciate any insight into the reason as it would help me make better contributions in the future. Thank you!