Update azure-pipelines.yml for Azure Pipelines

Author: Czhang0727

azure-pipelines.yml

@@ -27,7 +27,7 @@ steps:
       ssh-keyscan -t rsa github.com >> ~/.ssh/known_hosts
       git config --global user.email "[email protected]"
       git config --global user.name "Azure CI/CD Agent"
-      git submodule update 
+      git submodule update --init
       git submodule foreach git checkout master 
       git submodule foreach git pull origin master 
       hexo g