Fixed title level of "Validation rules" on custom-resource-definitions.md

The title level of "Validation rules" should be H3, instead of H2.

Author: able8

content/en/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions.md

@@ -769,7 +769,7 @@ validations are not supported by ratcheting under the implementation in Kubernet
   ratcheted.
 
 
-## Validation rules
+### Validation rules
 
 {{< feature-state state="beta" for_k8s_version="v1.25" >}}