Fix typos (#9488)

See https://github.com/zeit/next.js/pull/9109#issuecomment-557139022

examples/amp-first/components/Layout.js

@@ -27,7 +27,7 @@ const Layout = props => (
     <AmpIncludeAmpInstallServiceworker />
     <amp-install-serviceworker
       src="/serviceworker.js"
-      data-iframe-src="/install-sw.html"
+      data-iframe-src="/install-serviceworker.html"
       layout="nodisplay"
     />
 

examples/amp-first/public/install-serviceworker.html

@@ -2,6 +2,6 @@
 <title>installing service worker</title>
 <script type="text/javascript">
   if ('serviceWorker' in navigator) {
-    navigator.serviceWorker.register('/sw.js')
+    navigator.serviceWorker.register('/serviceworker.js')
   }
 </script>