Update nginx-metrics.json

Author: ishanjainn

nginx-mixin/dashboards/nginx-metrics.json

@@ -561,7 +561,7 @@
     ]
   },
   "timezone": "",
-  "title": "NGINX",
+  "title": "NGINX Overview",
   "uid": "MsjffzSZz",
   "version": 1
 }