fix ms.date format

Author: mairaw

docs/core/porting/libraries.md

@@ -2,7 +2,7 @@
 title: Port libraries to .NET Core
 description: Learn how to port library projects from the .NET Framework to .NET Core.
 author: cartermp
-ms.date: 12/7/2018
+ms.date: 12/07/2018
 ms.custom: seodec18
 ---
 # Port .NET Framework libraries to .NET Core