[Doctests] Correct filenaming
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Removes duplicated lines in doc tests file and adds leading language identifier. 

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
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Library:

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- tokenizers: @n1t0, @LysandreJik
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Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
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Comments:
This was probably forgotten in https://github.com/huggingface/transformers/pull/16518 cc @sgugger 

_The documentation is not available anymore as the PR was closed or merged._