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

OWNERS

@@ -1,7 +1,7 @@
-assignees:
+approvers:
   - mrunalp
   - runcom
   - rhatdan
   - nalind
   - mheon
-  - baude
+  - baude