Typo on doctring in ElectraTokenizer
Author: FacerAin
Pull request: yes

# What does this PR do?
Typo on docstring in [ElectraTokenizer](https://github.com/huggingface/transformers/blob/6cc06d17394f5715cdf2d13a1ef7680bedaee9e2/src/transformers/models/electra/tokenization_electra.py#L93).
This should be modified from **Bert** to **Electra** for readability.
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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20192). All of your documentation changes will be reflected on that endpoint.

Thank you for the guide when I was confused with the copy check. 😊
I add the part `,BERT->Electra` you said.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20192). All of your documentation changes will be reflected on that endpoint.