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.
@@ -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
```
