mark OIDC discovery feature as implemented

Author: pacoxu

keps/sig-auth/1393-oidc-discovery/kep.yaml

@@ -22,7 +22,7 @@ prr-approvers:
 editor: TBD
 creation-date: 2018-06-26
 last-updated: 2020-01-25
-status: implementable
+status: implemented
 replaces:
   - "https://github.com/kubernetes/community/pull/2314/"