Update toc

Author: cici37

keps/sig-api-machinery/3488-cel-admission-control/README.md

@@ -73,6 +73,7 @@
       - [e2e tests](#e2e-tests)
   - [Graduation Criteria](#graduation-criteria)
     - [Alpha](#alpha)
+    - [Beta](#beta)
   - [Upgrade / Downgrade Strategy](#upgrade--downgrade-strategy)
   - [Version Skew Strategy](#version-skew-strategy)
 - [Production Readiness Review Questionnaire](#production-readiness-review-questionnaire)