Fix: progressive-web-app example (#20288)

Fixes: #20289 
I noticed the following errors about missing icon files while working on the progressive-web-app example
<img width="639" alt="Screenshot 2020-12-18 at 9 16 25 AM" src="https://user-images.githubusercontent.com/11258286/102572751-e3ae4d80-4112-11eb-896a-2facfb5e16d3.png">

I've added the missing icon files and updated the path :)

examples/progressive-web-app/pages/_app.js

@@ -17,13 +17,13 @@ export default function MyApp({ Component, pageProps }) {
 
         <link rel="manifest" href="/manifest.json" />
         <link
-          href="/favicon-16x16.png"
+          href="/icons/favicon-16x16.png"
           rel="icon"
           type="image/png"
           sizes="16x16"
         />
         <link
-          href="/favicon-32x32.png"
+          href="/icons/favicon-32x32.png"
           rel="icon"
           type="image/png"
           sizes="32x32"