Fixed php error in file, usage of undefined variable

M1 team clearly did a typo with: $txnId

Author: manuperezgo

app/code/core/Mage/Sales/Model/Order/Payment/Transaction.php

@@ -277,7 +277,7 @@ public function getChildTransactions($types = null, $txnId = null, $recursive =
                 }
             } else {
                 foreach ($this->_children as $child) {
-                    if ($child->getTxnId() === $tnxId) {
+                    if ($child->getTxnId() === $txnId) {
                         $transaction = $child;
                         break;
                     }