theme/index.hbs: fix link to favicon

Author: paper42

src/theme/index.hbs

@@ -8,7 +8,7 @@
         <meta name="viewport" content="width=device-width, initial-scale=1">
         <meta name="theme-color" content="#ffffff" />
 
-        <link rel="shortcut icon" href="{{ path_to_root }}{{ favicon }}">
+        <link rel="shortcut icon" href="{{ path_to_root }}{{ favicon_png }}">
         <link rel="stylesheet" href="{{ path_to_root }}css/general.css">
         <link rel="stylesheet" href="{{ path_to_root }}css/print.css" media="print">
     </head>