Clarify step 3

Author: qbzzt

pages/stack/interop/superchain-erc20.mdx

@@ -61,7 +61,7 @@ sequenceDiagram
     note over src-erc20, dst-l2Xl2: Initiating Message
     user->>src-bridge: 1. Move n tokens
     src-bridge->>src-erc20: 2. Burn n tokens
-    src-bridge->>dst-l2Xl2: 3. Relay n tokens to user
+    src-bridge--)dst-l2Xl2: 3. Relay n tokens to user
   end
   rect rgba(0,0,0,0.1)
     note over user,dst-erc20: Executing message
@@ -79,6 +79,8 @@ sequenceDiagram
 
 3.  The source token bridge calls [`SuperchainTokenBridge.relayERC20`](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/src/L2/SuperchainTokenBridge.sol#L80-L97) on the destination token bridge.
     This call is relayed using [`L2ToL2CrossDomainMessenger`](./message-passing).
+    The call is *initiated* here, by emitting an initiating message. 
+    It will be executed later, after the destination chain receives an executing message to [`L2ToL2CrossDomainMessenger`](./message-passing).
 
 #### Executing message (destination chain)