Fix legacy book link
The domain alias isn't working quite right, so just point directly to
the canonical subdomain.
**Note:** Impatient readers may head straight to [Quick
Start](quick-start.md).
-**Using Kubebuilder v1? Check the [legacy documentation](https://v1.book.kubebuilder.io)**
+**Using Kubebuilder v1? Check the [legacy
+documentation](https://book-v1.book.kubebuilder.io)**
## Who is this for
