Fix markdown formatting in developer guide (#1426)

Signed-off-by: Ryan Bogan <[email protected]>
(cherry picked from commit d5538a4)

DEVELOPER_GUIDE.md

@@ -298,6 +298,7 @@ curl https://localhost:9200 --insecure -u admin:<admin-password>
   },
   "tagline" : "The OpenSearch Project: https://opensearch.org/"
 }
+```
 
 ### Run Multi-node Cluster Locally