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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
```