docs: fix type (#14064)

this slot name should be `footer` instead it was `header`

Author: Anyass3

documentation/docs/07-misc/07-v5-migration-guide.md

@@ -376,7 +376,7 @@ If you wanted multiple UI placeholders, you had to use named slots. In Svelte 5,
 </main>
 
 <footer>
-	---<slot name="header" />---
+	---<slot name="footer" />---
 	+++{@render footer()}+++
 </footer>
 ```