Update metadata for x_stance dataset
Great! Thanks @jvamvas for these updates.
 I have fixed a warning. The remaining test failure is due to an unrelated dataset. We just fixed the other dataset on master. Could you rebase from master and push to rerun the CI ?
Thank you for featuring the x_stance dataset in your library. This PR updates some metadata:
- Citation: Replace preprint with proceedings
- URL: Use a URL with long-term availability
