[DOC] correct docstring of BaseForecaster.score, reference to use of non-symmetric MAPE

[MBristle]

Reference Issues/PRs

Fixes #4947

What does this implement/fix? Explain your changes.

The default value of the mean_absolute_percentage_error function changed to the non-symmetric MAPE. This has been made explicit, and the doc-string is adjusted.

Does your contribution introduce a new dependency? If yes, which one?

none

What should a reviewer concentrate their feedback on?

Was the change from symmetric - MAPE to non-symmetric MAPE intended for the score function of the BaseForecaster?
How are down-stream tasks affected in either way?

Did you add any tests for the change?

Functionality was not changed.

Any other comments?

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[fkiraly]

Thanks for the fix!

One change request - the file CONTRIBUTORS.md should not be changed directly, this is automatically populated from .all-contributorsrc (so your change would be reverted at the next auto-update). Please change the latter, the all-contributorsrc, that's the "ground truth".

[fkiraly]

Looks good now!

[fkiraly]

Was the change from symmetric - MAPE to non-symmetric MAPE intended for
the score function of the BaseForecaster?

That's a good question - I think the default changed simply since the non-symmetric version is more common, and more in line with the user expectation when reading the class name. As long as the docstring is consistent, all should be fine?

Downstream tasks - like tuning - afaik are not affected as the user always specifies the metric themselves, and there the docstrings are correct.