[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.