[#616] fixed wrong file extension in index.html for manifest (#627)

public/index.html

@@ -6,11 +6,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1" />
     <meta name="theme-color" content="#000000" />
     <link rel="apple-touch-icon" href="<%= assetPrefix %>/calendar.svg" />
-    <!--
-      manifest.json provides metadata used when your web app is installed on a
-      user's mobile device or desktop. See https://developers.google.com/web/fundamentals/web-app-manifest/
-    -->
-    <link rel="manifest" href="<%= assetPrefix %>/manifest.json" />
+    <link rel="manifest" href="<%= assetPrefix %>/manifest.appdata" />
     <!--
       Notice the use of <%= assetPrefix %> in the tags above.
       It will be replaced with the URL of the `public` folder during the build.