Minor typo referencing RoBERTa
"Roberta" should use the correct capitalization: "RoBERTa"

https://github.com/huggingface/transformers/blob/d27e4c18fe2970abcb9a48dcb8a824e48083b15f/docs/source/en/tokenizer_summary.md?plain=1#L144

Should be a simple fix.