tweak screenshots

apps/rings/metadata.json

@@ -9,6 +9,8 @@
     "url": "screenshot1.png"
   }, {
     "url": "screenshot2.png"
+  }, {
+    "url": "screenshot3.png"  
   }],
   "type": "clock",
   "tags": "clock",
@@ -31,4 +33,4 @@
   "data": [{
     "name": "rings.settings.json"
   }]
-}
+}