Update etrog-upgrade.md

replacing Block Explorer link with the testnet polygonscan one https://cardona-zkevm.polygonscan.com/ ... as per Rucha's request here https://0xpolygon.slack.com/archives/C061MU1E309/p1706791098241629

Author: EmpieichO

docs/zkEVM/architecture/protocol/etrog-upgrade.md

@@ -26,7 +26,7 @@ Use the following details to connect wallets to Cardona:
 
 - Currency symbol: ETH
 
-- Block Explorer: https://explorer-ui.cardona.zkevm-rpc.com
+- Explorer: https://cardona-zkevm.polygonscan.com/
 
 We use the same faucet for testnet tokens, here: https://faucet.polygon.technology/