Update storage-classes.md

Using the documented allowed topologies config I get the following deprecation warning: 
```
Warning: allowedTopologies[0].matchLabelExpressions[0].key: deprecated since v1.17; use "topology.kubernetes.io/zone" instead
```

Using "topology.kubernetes.io/zone", I get successful deployment:
```
kubectl edit storageclass/gp2
storageclass.storage.k8s.io/gp2 edited
```

Author: mcameron

content/en/docs/concepts/storage/storage-classes.md

@@ -243,7 +243,7 @@ parameters:
 volumeBindingMode: WaitForFirstConsumer
 allowedTopologies:
 - matchLabelExpressions:
-  - key: failure-domain.beta.kubernetes.io/zone
+  - key: topology.kubernetes.io/zone
     values:
     - us-central-1a
     - us-central-1b