Fix image placeholder example

Author: surma

css-paint-api/Overview.bs

@@ -770,7 +770,8 @@ loaded.
 <script>
     CSS.registerProperty({
         name: '--image',
-        syntax: '<image>'
+        syntax: '<image> | none',
+        initialValue: 'none',
     });
     CSS.paintWorklet.addModule('image-placeholder.js');
 </script>