I know this is six months too late, but I just went through the same tutorial and found the problem. So, for any future people with the same issue, here is the answer:
Add “get_stylesheet_directory_uri()” to your link so it is pulling the favicon.ico from your child theme directory. You want it there and not in your parent theme folder. It’s best to keep everything separated.
So, the code that worked for me is:
function favicon_link() {
echo '<link rel="shortcut icon" type="image/x-icon" href="'.get_stylesheet_directory_uri().'/favicon.ico" />' . "\n";
}
add_action('wp_head', 'favicon_link');
