Segformer TF: fix output size in documentation
Fixes # (issue)
https://github.com/huggingface/transformers/issues/18557

## Before submitting
- [ X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?
@sayakpaul 


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_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi,

thanks for your PR. Could you also fix this in `modeling_segformer.py` for the PyTorch implementation?

Thanks!