update mtedx bibtex to conf proceedings (#3984)
update mtedx bibtex to conf proceedings (#3984)

Summary:
## What does this PR do?
Updates the bibtex to cite the mtedx dataset in the speech-to-text example, now that mtedx has been published (Interspeech 2021)

Pull Request resolved: https://github.com/pytorch/fairseq/pull/3984

Reviewed By: yuntang

Differential Revision: D32102153

Pulled By: kahne

fbshipit-source-id: 1eea532e9032c116da447a4da30669305559a787

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq