fix onConfigure access-modifier

Change `OnConfigure` access modifier to match the overridden method.


In this PR ChilliCream#5750,
you changed `OnConfigure` from `public` to `protected` and forgot to update it in the docs.

website/src/docs/hotchocolate/v12/api-reference/custom-attributes.md

@@ -129,7 +129,7 @@ It is super simple to create custom descriptor attributes and package complex fu
 public class SomeMiddlewareAttribute
     : ObjectFieldDescriptorAttribute
 {
-    public override void OnConfigure(
+    protected override void OnConfigure(
         IDescriptorContext context,
         IObjectFieldDescriptor descriptor,
         MemberInfo member)