Correct SerializingProducer Exception doc (#912)

Author: rnpridgeon

confluent_kafka/serializing_producer.py

@@ -157,7 +157,7 @@ def produce(self, topic, key=None, value=None, partition=-1,
             ValueSerializationError: If an error occurs during value
             serialization.
 
-             ProduceException: For all other errors
+            KafkaException: For all other errors
 
         """
         ctx = SerializationContext(topic, MessageField.KEY)