fix(combo): dropdown selector (#491)

Author: SisIvanova

sass/themes/components/combo/_combo-theme.scss

@@ -57,7 +57,7 @@
     $selector: (
         #{config.element-prefix() + '-' + 'combo'},
         #{config.element-prefix() + '-' + 'simple-combo'},
-        .igx-drop-down__list
+        '.igx-drop-down__list'
     );
     $combo-schema: ();