Update transaction signature return in README diagram

This commit updates the sequence diagram in the README file to clarify the step where the validator returns the transaction signature to the settlement. The previous description was misleading as it mentioned a 'signed approval' instead. This change improves accuracy and ensures clear understanding of the process.

README.md

@@ -86,8 +86,8 @@ sequenceDiagram
     Settlement->>Settlement: Generate proof
     Settlement->>Validator: Submit for validation
     Validator->>Validator: Verify state transition
-    Validator->>Settlement: Return signed approval
     Settlement->>MainChain: Submit new state root & signature
+    Validator->>Settlement: Return transaction signature
     MainChain->>MainChain: Verify validator's signature
     alt Signature is valid
         MainChain->>MainChain: Update state root