add doc link for popstate event

exercises/02.side-effects/01.problem.effects/README.mdx

@@ -1,6 +1,7 @@
 # useEffect
 
-👨‍💼 We want the query to update as the query params change through a `popstate`
+👨‍💼 We want the query to update as the query params change through a
+[`popstate`](https://developer.mozilla.org/en-US/docs/Web/API/Window/popstate_event)
 event. Once you added the `useEffect` hook, you then have to prevent the default
 full refresh of the page when the user clicks the back/forward button.