DR --> disaster recovery

I was unsure what DR means, and I guess it means "disaster recovery".

Let's make it explicit, so that nobody needs to look it up.

Author: guettli

docs/book/src/user/quick-start.md

@@ -30,7 +30,7 @@ a target [management cluster] on the selected [infrastructure provider].
 
 1. **Existing Management Cluster**
 
-   For production use-cases a "real" Kubernetes cluster should be used with appropriate backup and DR policies and procedures in place. The Kubernetes cluster must be at least v1.20.0.
+   For production use-cases a "real" Kubernetes cluster should be used with appropriate backup and disaster recovery policies and procedures in place. The Kubernetes cluster must be at least v1.20.0.
 
    ```bash
    export KUBECONFIG=<...>