Change annotation name to allow volume mode change

[RaunakShah]

What type of PR is this?
/kind feature

What this PR does / why we need it:

This PR updates the annotation that needs to be applies to VolumeSnapshotContents from snapshot.storage.kubernetes.io/allowVolumeModeChange to snapshot.storage.kubernetes.io/allow-volume-mode-change.
The feature is still in the Alpha stage.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Update the annotation that needs to be applies to VolumeSnapshotContents from snapshot.storage.kubernetes.io/allowVolumeModeChange to snapshot.storage.kubernetes.io/allow-volume-mode-change

[xing-yang]

In the release node, please clarify the old annotation and the new one.

[xing-yang]

Please also submit a PR to kubernetes-csi/docs to describe this feature.

[xing-yang]

I see that you've submitted a doc and it is also merged, but it is not shown up in here: https://kubernetes-csi.github.io/docs/features.html

Please also update this file to add an entry: https://raw.githubusercontent.com/kubernetes-csi/docs/master/book/src/SUMMARY.md

[xing-yang]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RaunakShah, xing-yang

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