Docs: Use a 180x180 apple-touch-icon.png.

Refs:
https://github.com/h5bp/html5-boilerplate/commit/1c4fcf32142e888946db579df0efbb52542e0ed1
https://github.com/h5bp/html5-boilerplate/commit/345925f04384b59929d8d39ba428cef548921408

[ci skip]

docs/_includes/header.html

@@ -45,7 +45,7 @@
 <![endif]-->
 
 <!-- Favicons -->
-<link rel="apple-touch-icon-precomposed" href="/apple-touch-icon-precomposed.png">
+<link rel="apple-touch-icon" href="/apple-touch-icon.png">
 <link rel="icon" href="/favicon.ico">
 
 <script>