Update elasticsearch.asciidoc (#4241)

Fix YAML formatting for `ouput.elasticsearch.index` option in example.

Author: endorama

docs/en/apm-server/legacy/copied-from-beats/outputs/elasticsearch/docs/elasticsearch.asciidoc

@@ -247,7 +247,7 @@ events into different indices:
 ------------------------------------------------------------------------------
 output.elasticsearch:
   hosts: ["http://localhost:9200"]
-  index: "apm-%{[observer.version]}-%{[processor.event]}-%{+yyyy.MM.dd}\" <1>
+  index: "apm-%{[observer.version]}-%{[processor.event]}-%{+yyyy.MM.dd}" <1>
 ------------------------------------------------------------------------------
 <1> +{beat_version_key}+ is a field managed by Beats that is added to every document;
 It holds the current version of APM Server. We recommend including