clarify chain id

Author: wilsoncusack

docs/base-account/guides/authenticate-users.mdx

@@ -68,7 +68,10 @@ try {
     params: [{
       version: '1',
       capabilities: {
-        signInWithEthereum: { nonce, chainId: '0x2105' }
+        signInWithEthereum: { 
+          nonce, 
+          chainId: '0x2105' // Base Mainnet - 8453
+        }
       }
     }]
   });