[pwa]: updated manifest reference in entry;

Author: maifeeulasad

index.html

@@ -14,7 +14,7 @@
       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="./manifest.json" />
+    <link rel="manifest" href="./manifest.webmanifest" />
     <!--
       Notice the use of %PUBLIC_URL% in the tags above.
       It will be replaced with the URL of the `public` folder during the build.