Documentation: RemBERT fixes
Hi,

this PR introduces some fixes for the RemBERT documentation:

Python Codeblock fix for the configuration class. It is currently broken:

![image](https://user-images.githubusercontent.com/20651387/172928831-444c9f6b-2df7-404d-a4cb-a6d5b628fe76.png)

And now fixed with this PR:

![image](https://user-images.githubusercontent.com/20651387/172931066-ab5ca258-b334-4154-befd-01b678e041b9.png)

See [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17641/en/model_doc/rembert#transformers.RemBertConfig.example) for live demo.

Additionally, the model identifier `rembert` is not working, because the model is stored under the `google` namespace. This fix was done passing `google/rembert` to the `checkpoint` argument of the `add_code_sample_docstrings` decorator. This was reported in #17147.

Old [model identifier example](https://huggingface.co/docs/transformers/v4.19.3/en/model_doc/rembert#transformers.RemBertModel.forward.example) vs. [new working model identifier example](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17641/en/model_doc/rembert#transformers.RemBertModel.forward.example).
_The documentation is not available anymore as the PR was closed or merged._<|||||>Good catch!<|||||>Thanks for fixing @stefan-it 