Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Replace storage-class annotations with field in examples
**What this PR does / why we need it**:
storage class is already GA. Replace annotations with field `StorageClassName` in examples.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes#51435 (update: thanks @gyliu513 for the issue)
ref: https://github.com/kubernetes/kubernetes/pull/50654#discussion_r134954171
**Special notes for your reviewer**:
We may also want to remove the beta annotations in 1.8 since the field will have already been in two releases. If @kubernetes/sig-storage-api-reviews confirm this, I'd like to help remove it.
/cc @liggitt @jsafrane @msau42
**Release note**:
```release-note
NONE
```
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