fix(menu-submenu): add parent class to set the initial display of the submenu to be open (#5022)

Co-authored-by: stephanie green <[email protected]>

Author: greensteph

ui/components/menus/docs.mdx

@@ -261,7 +261,7 @@ To open the submenu to the right of the main menu, add `slds-dropdown_submenu-ri
 
 <Example title="Menu with Submenu Right">
   <CodeView demoStyles="height: 300px;">
-    <Submenu ariaExpanded="true" />
+    <Submenu ariaExpanded parentIsOpen />
   </CodeView>
 </Example>
 
@@ -271,7 +271,7 @@ To open the submenu to the left, add `slds-dropdown_submenu-left` to the `<div>`
 
 <Example title="Menu with Submenu Left">
   <CodeView demoStyles="height: 300px; margin-left: 150px;">
-    <Submenu ariaExpanded submenuClassnames="slds-dropdown_submenu-left" />
+    <Submenu ariaExpanded parentIsOpen submenuClassnames="slds-dropdown_submenu-left" />
   </CodeView>
 </Example>