Rename OWNERS assignees: to approvers:

They are effectively the same, assignees is deprecated

docs/reference/api-overview.md

@@ -1,6 +1,6 @@
 ---
 title: Kubernetes API Overview
-assignees:
+approvers:
 - bgrant0607
 - erictune
 - lavalamp

docs/reference/client-libraries.md

@@ -1,6 +1,6 @@
 ---
 title: Client Libraries
-assignees:
+approvers:
 - ahmetb
 ---
 

docs/reference/deprecation-policy.md

@@ -1,5 +1,5 @@
 ---
-assignees:
+approvers:
 - bgrant0607
 - lavalamp
 - thockin

docs/reference/index.md

@@ -1,6 +1,6 @@
 ---
 title: Reference Documentation
-assignees:
+approvers:
 - chenopis
 ---
 

docs/reference/security.md

@@ -2,7 +2,7 @@
 layout: docwithnav
 title: Kubernetes Security and Disclosure Information
 permalink: /security/
-assignees:
+approvers:
 - eparis
 - erictune
 - philips