Update custom_models.mdx
Author: mishig25
Pull request: yes

BertModelForSequenceClassification -> [BertForSequenceClassification](https://huggingface.co/docs/transformers/main/en/model_doc/bert#transformers.BertForSequenceClassification)

# What does this PR do?

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Fixes # (issue)


## Before submitting
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Models:

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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)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

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Comments:
_The documentation is not available anymore as the PR was closed or merged._