Fix useMenu example (#2706)

Author: devongovett

packages/@react-aria/menu/docs/useMenu.mdx

@@ -426,17 +426,17 @@ function MenuItem({item, state, onAction}) {
   <Item textValue="Copy" key="copy">
     <div><strong>Copy</strong></div>
     <div>Copy the selected text</div>
-    <Keyboard>⌘C</Keyboard>
+    <kbd>⌘C</kbd>
   </Item>
   <Item textValue="Cut" key="cut">
     <div><strong>Cut</strong></div>
     <div>Cut the selected text</div>
-    <Keyboard>⌘X</Keyboard>
+    <kbd>⌘X</kbd>
   </Item>
   <Item textValue="Paste" key="paste">
     <div><strong>Paste</strong></div>
     <div>Paste the copied text</div>
-    <Keyboard>⌘V</Keyboard>
+    <kbd>⌘V</kbd>
   </Item>
 </Menu>
 ```