Docs(gossip): Broken link to plum tree documentation

[0xksure]

Problem

The link to Plum Tree protcol documentation under the gossip documentation is not correct anymore. The reason is that the domain owner redirects to Inesctec.

Proposed Solution

Change the link to correct documentation.

Maybe this is the correct link https://asc.di.fct.unl.pt/~jleitao/pdf/srds07-leitao.pdf ?

[github-actions]

This repository is no longer in use. Please re-open this issue in the agave repo: https://github.com/anza-xyz/agave