[DOC] Add one more condition in source field

[insukcho]

If source field is the same as the target field, the pipeline will crash with the error something like the below:

[ERROR][logstash.javapipeline    ][main] Pipeline worker error, the pipeline will be stopped {:pipeline_id=>"main", :error=>"Could not set field 'name' on object 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:45.0) Gecko/20100101 Firefox/45.0' to value 'Firefox'.This is probably due to trying to set a field like [foo][bar] = someValuewhen [foo] is not either a map or a string",

Thanks for contributing to Logstash! If you haven't already signed our CLA, here's a handy link: https://www.elastic.co/contributor-agreement/