Fix `TrainingArguments.label_names` docs to reflect the correct default value behaviour
Fixes #21287
@sgugger

_The documentation is not available anymore as the PR was closed or merged._<|||||>This was never flagged as a breaking change (indeed I only found this because it broke one of my scripts). I wonder if I should add "🚨 🚨 🚨" to the PR name to indicate a breaking change<|||||>The breaking change is in the PR that changed the default to `label_names` a while ago, not in this one :-) 