Update version for PriorityClass

[hardikdr]

With K8s 1.11 it should be scheduling.k8s.io/v1beta1

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[tengqm]

/lgtm
Encountered this when reworking the API reference doc generator.
/assign @MistyHacks

[mdlinville]

/approve

@tengqm Ideally when would this have been caught and by whom, during the 1.11 cycle?

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[tengqm]

@MistyHacks I only found this when I was working on the "native" API reference doc generator (kubernetes-sigs/reference-docs#53). I copied the OpenAPI JSON file out of kubernetes/kubernetes release-1.11 branch and ran it against the doc generator, and I got an error that the PriorityClass beta version is not found in TOC (thanks to the cross checking logic in the generator). I have no idea how this happened because we already regenerated the API ref doc when release-1.11 was out.