OWNERS: rename 'assignees' to 'approvers'

As CRI-O did in [1].  'assignees' is deprecated and 'approvers' is
effectively the same thing [2].

[1]: https://github.com/kubernetes-incubator/cri-o/pull/1103
[2]: https://github.com/kubernetes/test-infra/issues/3851

Signed-off-by: W. Trevor King <wking@tremily.us>

Closes: #688
Approved by: rhatdan
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