Adding documentation on how to overload `SacreBLEU` arguments
Author: Narsil
Pull request: yes

in the example


# What does this PR do?

Fixes #14758

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Fixes # (issue)


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Comments:
This seems like a very niche use case so let's not clutter the example README with it. We can add a comment in the actual script if we feel it's really necessary, but we already say everywhere that those examples are **just examples** and that users should customize them to their needs.

I am perfectly fine not adding that. 
I will drop that PR then. (People will most likely be referred to the original issue on google I guess)