docs: fix indentation in CephCluster on PVC example yaml

Copy pasting current code block results in an YAML parsing error and
this change fixing that error by adding a space where its needed

Signed-off-by: Tarun Gupta Akirala <takirala@users.noreply.github.com>

Documentation/CRDs/Cluster/pvc-cluster.md

@@ -30,7 +30,7 @@ spec:
           requests:
             storage: 10Gi
   storage:
-   storageClassDeviceSets:
+    storageClassDeviceSets:
     - name: set1
       count: 3
       portable: false