fix dead link

Author: yyx990803

src/guide/essentials/watchers.md

@@ -399,7 +399,7 @@ export default {
 
 But what if `id` changes before the request completes? When the previous request completes, it will still fire the callback with an ID value that is already stale. Ideally, we want to be able to cancel the stale request when `id` changes to a new value.
 
-We can use the [`onWatcherCleanup()`](/api/reactivity-core/#onwatchercleanup) <sup class="vt-badge" data-text="3.5+" /> API to register a cleanup function that will be called when the watcher is invalidated and is about to re-run:
+We can use the [`onWatcherCleanup()`](/api/reactivity-core#onwatchercleanup) <sup class="vt-badge" data-text="3.5+" /> API to register a cleanup function that will be called when the watcher is invalidated and is about to re-run:
 
 <div class="composition-api">