fix: improve popover child animation example for Safari

Author: kp047i

packages/react-aria-components/stories/Popover.stories.tsx

@@ -456,6 +456,11 @@ export const PopoverChildElementAnimationExample = () => (
           }}>
           <style>
             {`
+              @starting-style {
+                .react-aria-Popover .content {
+                  opacity: 0;
+                }
+              }
               .react-aria-Popover[data-exiting] .content,
               .react-aria-Popover[data-entering] .content {
                 opacity: 0;