Update README.md to reflect correct tag key for snapshots

[rdpsin]

Is this a bug fix or adding new feature?
Doc fix. Fixes #1185

What is this PR about? / Why do we need it?
The docs mention that the driver applies CSISnapshotName as the tag
key for snapshots.

However, we apply CSIVolumeSnapshotName as the tag key.

This change fixes README.md to reflect the correct tag key.

What testing is done?

[k8s-ci-robot]

Hi @rdpsin. Thanks for your PR.

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[gtxu]

/ok-to-test

[gtxu]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gtxu, rdpsin

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