[MINOR:TYPO] Update base.py
Author: cakiki
Pull request: yes

All other occurrences in the docs use lowercase. (https://github.com/search?q=repo%3Ahuggingface%2Ftransformers%20translation_XX_to_YY&type=code)

Also, using uppercase doesn't work: tested with "translation_EN_to_FR" which doesn't work and instead returns:  `ValueError: The task does not provide any default models for options ('EN', 'FR')`

It might be a good idea to allow for uppercase, but that's for another issue.


cc @stevhliu


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