Update exception docstring

Author: Andrew Kowalik

kafka/producer/kafka.py

@@ -584,7 +584,7 @@ def _wait_on_metadata(self, topic, max_wait):
             set: partition ids for the topic
 
         Raises:
-            TimeoutException: if partitions for topic were not obtained before
+            KafkaTimeoutError: if partitions for topic were not obtained before
                 specified max_wait timeout
         """
         # add topic to metadata topic list if it is not there already.