Menu: fix default-openeds description (#10316)

which should be indexes of the sub-menu

examples/docs/en-US/menu.md

@@ -304,7 +304,7 @@ Vertical NavMenu could be collapsed.
 | text-color  | text color of Menu (hex format) | string |   —   | #303133 |
 | active-text-color  | text color of currently active menu item (hex format) | string |   —   | #409EFF |
 | default-active | index of currently active menu | string    | — | — |
-| default-openeds | array that contains keys of currently active sub-menus  | Array    | — | — |
+| default-openeds | array that contains indexes of currently active sub-menus  | Array    | — | — |
 | unique-opened  |  whether only one sub-menu can be active  | boolean   | — | false   |
 | menu-trigger | how sub-menus are triggered, only works when `mode` is 'horizontal' | string    | — | hover |
 | router  | whether `vue-router` mode is activated. If true, index will be used as 'path' to activate the route action | boolean   | — | false   |

examples/docs/zh-CN/menu.md

@@ -305,7 +305,7 @@
 | text-color  | 菜单的文字颜色（仅支持 hex 格式） | string |   —   | #303133 |
 | active-text-color  | 当前激活菜单的文字颜色（仅支持 hex 格式） | string |   —   | #409EFF |
 | default-active | 当前激活菜单的 index | string    | — | — |
-| default-openeds | 当前打开的sub-menu的 key 数组 | Array    | — | — |
+| default-openeds | 当前打开的 sub-menu 的 index 的数组 | Array    | — | — |
 | unique-opened  | 是否只保持一个子菜单的展开 | boolean   | — | false   |
 | menu-trigger  | 子菜单打开的触发方式(只在 mode 为 horizontal 时有效) | string   | — | hover   |
 | router  | 是否使用 vue-router 的模式，启用该模式会在激活导航时以 index 作为 path 进行路由跳转 | boolean   | — | false   |