Fix example of custom Trainer to reflect signature of compute_loss
Author: lewtun
Pull request: yes

# What does this PR do?

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Fixes out-of-date example of custom `Trainer` in docs. 

Since several people have asked about multi-label classification in the forum and in #10232 I thought it might be useful to use this as the example.

I also took the liberty of tightening the grammar a bit in the preceding text 😃 


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [ ] Did you write any new necessary tests?

Forum link: https://discuss.huggingface.co/t/custom-loss-compute-loss-got-an-unexpected-keyword-argument-return-outputs/4148?u=lewtun

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Comments:
Not sure why the tests are failing since I only tweaked the docs - perhaps it's a problem with the CI on your end?