Change template manifest start_url to dot to make it react-router friendly

Author: Rami Keränen

Diff for: packages/react-scripts/template/public/manifest.json

@@ -8,7 +8,7 @@
       "type": "image/png"
     }
   ],
-  "start_url": "./index.html",
+  "start_url": ".",
   "display": "standalone",
   "theme_color": "#000000",
   "background_color": "#ffffff"