fix(dock-manager): typo made in the fluent schema

Author: simeonoff

projects/igniteui-angular/src/lib/core/styles/themes/schemas/dark/_dock-manager.scss

@@ -141,7 +141,7 @@ $_dark-fluent-dock-manager: extend(
         joystick-icon-color-active: (
             igx-contrast-color: 'surface',
         ),
-        pane-header-backround: (
+        pane-header-background: (
             igx-color: 'surface',
             rgba: .85,
             hexrgba: #000,