[hotfix] Fix misleading log message in setTransactionId

Author: Savonitar

flink-connector-kafka/src/main/java/org/apache/flink/connector/kafka/sink/internal/FlinkKafkaInternalProducer.java

@@ -179,7 +179,7 @@ public long getProducerId() {
     public void setTransactionId(String transactionalId) {
         checkState(
                 !isInTransaction(),
-                String.format("Another transaction %s is still open.", transactionalId));
+                String.format("Another transaction %s is still open.", this.transactionalId));
         LOG.debug("Change transaction id from {} to {}", this.transactionalId, transactionalId);
         this.transactionalId = transactionalId;
         Object transactionManager = getTransactionManager();