Fix typo distilbert doc to code link
Author: flozi00
Pull request: yes

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Comments:
Should fix https://github.com/huggingface/transformers/issues/13638

What about https://github.com/huggingface/transformers/blob/master/docs/source/model_doc/retribert.rst ?
Linking to same Distil folder path, what's the correct one it should point to ?