Update docs for elasticsearch filter plugin

[ctindel]

This doc:

https://www.elastic.co/guide/en/logstash/current/plugins-filters-elasticsearch.html

Needs to be updated to reflect the breaking changes from 5.0 to use event.get() and event.set() in the ruby code, otherwise you'll get exceptions like this:

[2016-12-23T05:00:11,456][ERROR][logstash.filters.ruby ] Ruby exception occurred: Direct event field references (i.e. event['field']) have been disabled in favor of using event get and set methods (e.g. event.get('field')). Please consult the Logstash 5.0 breaking changes documentation for more details.

This is discussed here:

https://www.elastic.co/guide/en/logstash/current/breaking-changes.html

[suyograo]

Fixed in logstash-plugins/logstash-filter-elasticsearch@55461d6