Update powershell-core.yml (#10363)

* Update powershell-core.yml

* Update powershell-core.yml

Author: dingmeng-xue

.azure-pipelines/powershell-core.yml

@@ -1,8 +1,8 @@
 variables:
   WindowsName: windows
-  WindowsImage: VS2017-Win2016
+  WindowsImage: vs2017-win2016
   LinuxName: linux
-  LinuxImage: Ubuntu-18.04
+  LinuxImage: ubuntu-18.04
   MacOSName: macOS
   MacOSImage: macOS-10.13
   TestFramework: netcoreapp2.1