docs/postmortems: fix outdated link (#28893)

Author: keienWang

docs/postmortems/2021-08-22-split-postmortem.md

@@ -87,7 +87,7 @@ The blocks on the 'bad' chain were investigated, and Tim Beiko reached out to th
 
 ### Disclosure decision
 
-The geth-team have an official policy regarding [vulnerability disclosure](https://geth.ethereum.org/docs/vulnerabilities/vulnerabilities). 
+The geth-team have an official policy regarding [vulnerability disclosure](https://geth.ethereum.org/docs/developers/geth-developer/disclosures). 
 
 > The primary goal for the Geth team is the health of the Ethereum network as a whole, and the decision whether or not to publish details about a serious vulnerability boils down to minimizing the risk and/or impact of discovery and exploitation.