Fix state root submission actor in README diagram

Changed the actor responsible for submitting the new state root and signature from Settlement to Validator. This correction aligns the documentation with the actual process flow.

README.md

@@ -86,7 +86,7 @@ sequenceDiagram
     Settlement->>Settlement: Generate proof
     Settlement->>Validator: Submit for validation
     Validator->>Validator: Verify state transition
-    Settlement->>MainChain: Submit new state root & signature
+    Validator->>MainChain: Submit new state root & signature
     Validator->>Settlement: Return transaction signature
     MainChain->>MainChain: Verify validator's signature
     alt Signature is valid