Update _build.yml

Looks like the Azure Pipelines are failed due to the image "Ubuntu 16.04" no longer available, suggest change the image to "ubuntu-latest", please review.

Author: QijieXue

_build.yml

@@ -5,7 +5,7 @@ jobs:
     workspace:
       clean: all
     pool:
-      vmImage: 'Ubuntu 16.04'
+      vmImage: 'ubuntu-latest'
     container:
       image: 'microsoftlearning/markdown-build:latest'
     steps: