fixed broken link
Author: VpkPrasanna
Pull request: yes

# What does this PR do?
Fixed the broken link for flatten method of dataset process


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Fixes # (issue)
![Screenshot from 2023-11-17 18-05-17](https://github.com/huggingface/transformers/assets/30804112/dbbbaf56-dbda-4305-9ef8-5ea9994f237b)

the link is 404 due to URL is wrong with .html at the end.
![Screenshot from 2023-11-17 18-06-20](https://github.com/huggingface/transformers/assets/30804112/d4be454a-977e-42b7-bdd9-936632f21b2a)


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## Who can review?
@stevhliu and @MKhalusova 


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