Fix CSS comment style to comply with styleguide

wagtail/wagtailadmin/static_src/wagtailadmin/scss/_mixins.scss

@@ -30,10 +30,11 @@
     }
 }
 
-@mixin unlistimmediate() {
-    /* remove list styles, but only for the immediate element - allow nested lists inside it
-    to keep the default style */
 
+// remove list styles, but only for the immediate element -
+// allow nested lists inside it to keep the default style
+
+@mixin unlistimmediate() {
     margin-top: 0;
     margin-bottom: 0;
     padding-left: 0;