chore(docs): add link to onError

Closes #1973 (#1974)

Author: dancrumb

Diff for: docs/en/advanced/navigation-guards.md

@@ -32,7 +32,7 @@ Every guard function receives three arguments:
 
   - **`next('/')` or `next({ path: '/' })`**: redirect to a different location. The current navigation will be aborted and a new one will be started. You can pass any location object to `next`, which allows you to specify options like `replace: true`, `name: 'home'` and any option used in [`router-link`'s `to` prop](../api/router-link.md) or [`router.push`](../api/router-instance.md#methods)
 
-  - **`next(error)`**: (2.4.0+) if the argument passed to `next` is an instance of `Error`, the navigation will be aborted and the error will be passed to callbacks registered via `router.onError()`.
+  - **`next(error)`**: (2.4.0+) if the argument passed to `next` is an instance of `Error`, the navigation will be aborted and the error will be passed to callbacks registered via [`router.onError()`](../api/router-instance.html#methods).
 
 **Make sure to always call the `next` function, otherwise the hook will never be resolved.**