Add missing PLBart entry in README
# What does this PR do?
This PR fixes the missing entry for PLBart (See #13269) in the index.

@sgugger @patil-suraj @patrickvonplaten @LysandreJik 
_The documentation is not available anymore as the PR was closed or merged._<|||||>I don't see the change in the main README. Also it's not part of the doc, so you have to use the full link to the master doc.