Update slashing.rs (#8676)

Link was a dead-end

substrate/frame/staking/src/slashing.rs

@@ -47,7 +47,7 @@
 //! has multiple misbehaviors. However, accounting for such cases is necessary
 //! to deter a class of "rage-quit" attacks.
 //!
-//! Based on research at <https://research.web3.foundation/en/latest/polkadot/slashing/npos/>
+//! Based on research at <https://w3f-research.readthedocs.io/en/latest/polkadot/slashing/npos.html>
 
 use super::{
 	EraIndex, Config, Module, Store, BalanceOf, Exposure, Perbill, SessionInterface,