Typo fix in document (#3764)

docs/en/setup/backend/backend-storage.md

@@ -87,7 +87,7 @@ storage:
 ### Data TTL
 TTL in ElasticSearch overrides the settings of core, read [ElasticSearch section in TTL document](ttl.md#elasticsearch-6-storage-ttl)
 
-### Elasticearch server settings
+### ElasticSearch server settings
 Read the [ElasticSearch storage FAQ](../../FAQ/ES-Server-FAQ.md) if you are new to ElasticSearch. 
 And recommend read more about these configuration from ElasticSearch official document. 
 This effects the performance of ElasticSearch very much.