tx-signing-example: Use emitOffchainLogs modifier (#63)

* use emitOffchainLogs modifier

* format

examples/std-transaction-signing/transaction-signing.sol

@@ -4,15 +4,14 @@ pragma solidity ^0.8.8;
 import "suave-std/suavelib/Suave.sol";
 import "suave-std/Transactions.sol";
 import "suave-std/Context.sol";
+import "suave-std/Suapp.sol";
 
-contract TransactionSigning {
+contract TransactionSigning is Suapp {
     using Transactions for *;
 
     event TxnSignature(bytes32 r, bytes32 s);
 
-    function callback(bytes32 r, bytes32 s) public {
-        emit TxnSignature(r, s);
-    }
+    function callback() external emitOffchainLogs {}
 
     function example() public returns (bytes memory) {
         string memory signingKey = string(Context.confidentialInputs());
@@ -29,6 +28,8 @@ contract TransactionSigning {
 
         Transactions.EIP155 memory txn = Transactions.signTxn(txnWithToAddress, string(signingKey));
 
-        return abi.encodeWithSelector(this.callback.selector, txn.r, txn.s);
+        emit TxnSignature(txn.r, txn.s);
+
+        return abi.encodeWithSelector(this.callback.selector);
     }
 }