Fix incorrect example in installation README

Currently there is no `stable` overlay available, example in README should use [`stable-master`](https://github.com/kubernetes-sigs/gcp-compute-persistent-disk-csi-driver/tree/master/deploy/kubernetes/overlays/stable-master) instead.

Author: PaulCapestany

docs/kubernetes/user-guides/driver-install.md

@@ -50,7 +50,7 @@ specified service account
 
 ```console
 $ GCE_PD_SA_DIR=/my/safe/credentials/directory    # Directory to get the service account key
-$ GCE_PD_DRIVER_VERSION=stable                    # Driver version to deploy
+$ GCE_PD_DRIVER_VERSION=stable-master             # Driver version to deploy
 $ ./deploy/kubernetes/deploy-driver.sh
 ```