Fix tokenizer class for `XLMRobertaXL`
# What does this PR do?

The 2 checkpoints for `XLMRobertaXL` use `XLMRobertaTokenizer` as `tokenizer_class`. In my PR #16857, I only checked `_CONFIG_FOR_DOC` in the modeling file at that time, and took `RobertaTokenizer` from there, which was wrong.
_The documentation is not available anymore as the PR was closed or merged._