[EncoderDecoder] fix encoder decoder config model type bug
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Small typo in config encoder-decoder class which leads to false config model type name


## Before submitting
- [ ] 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?


## Who can review?

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 albert, bert, XLM: @LysandreJik
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