Update silence_errors_in_log example

The example is unfortunately deprecated and does not work anymore. See logstash-plugins/logstash-output-elasticsearch#1159 

Also it would be nice to have a list of possible values somewhere - I could not find any

docs/plugins/outputs/elasticsearch.asciidoc

@@ -987,12 +987,12 @@ if enabled, script is in charge of creating non-existent document (scripted upda
 
 Defines the list of Elasticsearch errors that you don't want to log.
 A useful example is when you want to skip all 409 errors
-which are `document_already_exists_exception`.
+which are `version_conflict_engine_exception`.
 
 [source,ruby]
     output {
       elasticsearch {
-        silence_errors_in_log => ["document_already_exists_exception"]
+        silence_errors_in_log => ["version_conflict_engine_exception"]
       }
     }