[SPARK-6111] Fixed usage string in documentation.

Usage info in documentation does not match actual usage info.

Doc string usage says ```Usage: network_wordcount.py <zk> <topic>``` whereas the actual usage is ```Usage: kafka_wordcount.py <zk> <topic>```

Author: Kenneth Myers <myerske@us.ibm.com>

Closes #4852 from kennethmyers/kafka_wordcount_documentation_fix and squashes the following commits:

3855325 [Kenneth Myers] Fixed usage string in documentation.

examples/src/main/python/streaming/kafka_wordcount.py

@@ -17,7 +17,7 @@
 
 """
  Counts words in UTF8 encoded, '\n' delimited text received from the network every second.
- Usage: network_wordcount.py <zk> <topic>
+ Usage: kafka_wordcount.py <zk> <topic>
 
  To run this on your local machine, you need to setup Kafka and create a producer first, see
  http://kafka.apache.org/documentation.html#quickstart