Changed loudness theme URI to point to github (#6723)

loudness/style.css

@@ -1,6 +1,6 @@
 /*
 Theme Name: Loudness
-Theme URI: https://loudnessthemedemo.wordpress.com/
+Theme URI: https://github.com/Automattic/themes/tree/trunk/loudness
 Author: Automattic
 Author URI: https://automattic.com
 Description: A bold opinionated theme for music and learning