[Docs] Fix the paper URL for MMS model
# What does this PR do?

Fixes the paper link for the mms model, the wrong URL is for the paper `XLS-R: SELF-SUPERVISED CROSS-LINGUAL SPEECH REPRESENTATION LEARNING AT SCALE` not `Scaling Speech Technology to 1,000+ Languages`.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for the quick review! @amyeroberts Would you please merge it?