Minor fix on README.md
# What does this PR do?

Minor fix on README.md, including
- Think we want to provide links to arXiv paper info. pages, not the direct PDF file links (?).
- Paper title for `RoBERTa`
- Remove ending `)` in `ViTMAE` and `ViLT`.

@LysandreJik @NielsRogge @sgugger 
_The documentation is not available anymore as the PR was closed or merged._