Fix the name of the variable for the black image

Signed-off-by: Joas Schilling <coding@schilljs.com>

css/style.scss

@@ -510,7 +510,7 @@ video {
 	}
 
 	&.icon-menu-people {
-		background-image: var(--icon-menu-people-000);
+		background-image: var(--icon-spreed-menu-people-000);
 	}
 	&.icon-fullscreen {
 		background-image: var(--icon-fullscreen-000);