Replaced double quotes with single quotes

Using double quotes produces the error `[record accessor] syntax error, unexpected end of file, expecting STRING or INTEGER at '$kubernetes['`

Signed-off-by: sihoran <[email protected]>

Author: sihoran

pipeline/outputs/elasticsearch.md

@@ -250,8 +250,8 @@ The following snippet demonstrates using the namespace name as extracted by the
     Match *
     # ...
     Logstash_Prefix logstash
-    Logstash_Prefix_Key $kubernetes["namespace_name"]
+    Logstash_Prefix_Key $kubernetes['namespace_name']
     # ...
 ```
 
-For records that do nor have the field `kubernetes.namespace_name`, the default prefix, `logstash` will be used.
+For records that do nor have the field `kubernetes.namespace_name`, the default prefix, `logstash` will be used.