example(plugin.json): add dashboards array in includes

examples/nginx-app/plugin.json

@@ -34,10 +34,13 @@
     "updated": "2015-02-10"
   },
 
-  "includes": [
-    {"type": "panel", "name": "Nginx Panel"},
-    {"type": "datasource", "name": "Nginx Datasource"}
-  ],
+  "includes": {
+    "dashboards": [],
+    "plugins": [
+      {"type": "panel", "name": "Nginx Panel"},
+      {"type": "datasource", "name": "Nginx Datasource"}
+    ]
+  },
 
   "dependencies": {
     "grafanaVersion": "3.x.x",