fix: S2 docs Skeleton page (#9008)

Author: snowystinger

packages/dev/s2-docs/pages/s2/Skeleton.mdx

@@ -22,7 +22,7 @@ import Select from '@react-spectrum/s2/icons/Select';
       src={preview}
       width={1600}
       height={1200}
-      styles={style({height: 160, borderRadius: 'default', flexShrink: 0})} />
+      styles={style({height: 160, borderRadius: 'default', flexShrink: 0, aspectRatio: '4 / 3'})} />
     <div className={style({display: 'flex', flexDirection: 'column', gap: 16})}>
       <Text styles={style({font: 'heading-lg'})}>Placeholder title</Text>
       <Text styles={style({font: 'body'})}>This is placeholder content approximating the length of the real content to avoid layout shifting when the real content appears.</Text>