doc: fix ActiveLink detection condition (vercel#23472)

## Documentation / Examples

- [x] Make sure the linting passes


`router.pathname` can be dynamic routes like `/posts/[slug]`, use `router.asPath` instead to detect anchor is active

docs/api-reference/next/router.md

@@ -17,7 +17,7 @@ function ActiveLink({ children, href }) {
   const router = useRouter()
   const style = {
     marginRight: 10,
-    color: router.pathname === href ? 'red' : 'black',
+    color: router.asPath === href ? 'red' : 'black',
   }
 
   const handleClick = (e) => {