Made documentation consistent with other outputs that provide message_format such as pipe.
Made documentation consistent with other outputs that provide message_format such as pipe.

opentable/logstash-kafka,joekiller/logstash-kafka,Aloomaio/logstash-kafka,Aloomaio/logstash-kafka,opentable/logstash-kafka,joekiller/logstash-kafka,tinglinux/logstash-kafka,tinglinux/logstash-kafka