Fix the CSS for groove_simple

Previously, the css pointed to a non-existent
file path for the background.

airsonic-main/src/main/webapp/style/groove_simple.css

@@ -17,7 +17,7 @@
 
 /* Main frame image */
 .mainframe {
-    background-image:url( "../icons/groove_simple/background_main_blank.png" );
+    background-image:url( "../icons/groove/background_main_blank.png" );
 }