Update deletion policy in snapshot doc

[jingxu97]

Since Kubernetes v 1.13, VolumeSnapshotContent and VolumeSnapshotClass support deletion policy. Should update the doc here https://kubernetes.io/docs/concepts/storage/volume-snapshot-classes/

[xing-yang]

@jingxu97 Submitted doc PR: kubernetes/website#14719

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[msau42]

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