Improve test instructions in developer guide (opensearch-project#18736)

Signed-off-by: Shahbaz Aamir <shahbaz07dbit@gmail.com>

Author: shahbazaamir

DEVELOPER_GUIDE.md

@@ -189,7 +189,9 @@ It's typically easier to wait until the console stops scrolling, and then run `c
 
 ```bash
 curl localhost:9200
-
+```
+The expected reponse should be
+```
 {
   "name" : "runTask-0",
   "cluster_name" : "runTask",