Fix favicon sizes value in manifest (#3287)

Fixes #3284.

Author: ryansully

packages/react-scripts/template/public/manifest.json

@@ -4,7 +4,7 @@
   "icons": [
     {
       "src": "favicon.ico",
-      "sizes": "192x192",
+      "sizes": "64x64",
       "type": "image/png"
     }
   ],