update metadata for windows-node-limitations.md

Updating document date to reflect latest revision date of this file.
Given that the page is titled 'Limitations for Windows Server node pools in Azure Kubernetes Service (AKS)', I think it's important to have the last revision date displayed accurately to the reader.  (Otherwise they will assume that the page has not been edited for 9 months.)

Author: larryclaman

articles/aks/windows-node-limitations.md

@@ -6,7 +6,7 @@ author: mlearned
 
 ms.service: container-service
 ms.topic: article
-ms.date: 05/31/2019
+ms.date: 12/18/2019
 ms.author: mlearned
 
 #Customer intent: As a cluster operator, I want to understand the current limitations when running Windows node pools and application workloads.