fix plbart doctest
# What does this PR do?

We need to update the expect output in doc example after PR #19980
_The documentation is not available anymore as the PR was closed or merged._<|||||>@ydshieh @sgugger I just want to know how this PR works and why was the doctests failing earlier? Thanks in advance!<|||||>As the PR description mentioned, PR #19980 changed `PLBartTokenizer`, and some expected outputs in the tests have to be updated.<|||||>@ydshieh got it. Thanks! 