Change 'type' to 'item' for menu separator (#3726)

Author: patriacaelum

src/content/docs/learn/window-menu.mdx

@@ -36,7 +36,7 @@ const menu = await Menu.new({
       checked: true,
     },
     {
-      type: 'Separator',
+      item: 'Separator',
     },
     {
       id: 'disabled_item',