docs(modal): Fix en demo (#980)

Switching the messages, so that the one you'd expect shows up when clicking on "Submit" and "Cancel"

src/modal/demos/enUS/mask-closable.demo.md

@@ -29,11 +29,11 @@ export default defineComponent({
     return {
       showModal: showModalRef,
       onPositiveClick () {
-        message.success('Cancel')
+        message.success('Submit')
         showModalRef.value = false
       },
       onNegativeClick () {
-        message.success('Submit')
+        message.success('Cancel')
         showModalRef.value = false
       }
     }