Clarify error message for ChrF no. references

[BramVanroy]

Currently, when you give a different number of references and hypotheses to chrf, you get this error:

Sacrebleu requires the same number of references for each prediction

When you are only calculate chrf, it is clear that the error comes from calculating chrf. But if you do something like

evaluate.combine(["chrf", "bleu"])

it takes a moment to realize that chrf is causing the error and not bleu. Clarifying/reminding the user that chrf is implemented through sacrebleu can help.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.