fix(QBtnDropdown): API -> description for 'cover' property (#18208)

* Fix description for 'cover' property in QBtnDropdown

There is no `menu-fit` attribute for the QBtnDropdown

* Fix typo in QBtnDropdown.json description

Author: HannesOberreiter

ui/src/components/btn-dropdown/QBtnDropdown.json

@@ -63,7 +63,7 @@
 
     "cover": {
       "type": "Boolean",
-      "desc": "Allows the menu to cover the button. When used, the 'menu-self' and 'menu-fit' props are no longer effective",
+      "desc": "Allows the menu to cover the button. When used, the 'menu-self' prop is no longer effective",
       "category": "position"
     },