link to avalanche paper is outdated

[sbellem]

The current link is https://avalanchelabs.org/QmT1ry38PAmnhparPUmsUNHDEGHQusBLD6T5XJh4mUUn3v.pdf and redirects to https://www.avalabs.org/.

Use https://arxiv.org/abs/1906.08936 or https://assets.website-files.com/5d80307810123f5ffbb34d6e/6009805681b416f34dcae012_Avalanche%20Consensus%20Whitepaper.pdf.

The arxiv one seems less likely to change.