Fix typo in toctree
Author: mishig25
Pull request: yes

# What does this PR do?

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Fixes # (issue)


## Before submitting
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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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Comments:
regarding this conversation, every new doc page needs to be added to _toctree.yml (it serves the same purpose as this section of previous index.rst)
https://github.com/huggingface/transformers/blame/fc1d97f29d7b98e82ae17fc5ac49229e2859bcca/docs/source/index.rst#L603-L652
we can probably create some automatic script that adds a new model to toctree
@sgugger @LysandreJik 

Indeed, @sgugger was mentioning the same just yesterday :)

Yes, I plan to add a failure to the doc-builder if there is a MDX generated that ends up not being in the index (like sphinx used to do). Finishing the conversion to notebook and will add that next :-)