Fix broken Avalanche Consensus white paper link

[martineckardt]

The link pointing to avalabs.org is broken, since the file has been moved to a new domain. Therefore, I propose changing the link to arXiv link.

If it's desired to use the same link used on the Ava Labs website this would be it: https://assets-global.website-files.com/5d80307810123f5ffbb34d6e/6009805681b416f34dcae012_Avalanche%20Consensus%20Whitepaper.pdf