Fix XML for workflow

It wasn't generating a valid XML, attribute name was missing.

Change-Id: I2c0e28e4f12376cb74d92b261bb3f13cf70f424d
diff --git a/jenkins/files/jobs/workflow.xml b/jenkins/files/jobs/workflow.xml
index 561d6f2..43f97a5 100644
--- a/jenkins/files/jobs/workflow.xml
+++ b/jenkins/files/jobs/workflow.xml
@@ -14,7 +14,7 @@
 {%- endmacro -%}
 
 <?xml version='1.0' encoding='UTF-8'?>
-<flow-definition ="workflow-job@{{ salt['grains.get']('jenkins_plugins:workflow-job:version', '2.5') }}">
+<flow-definition plugin="workflow-job@{{ salt['grains.get']('jenkins_plugins:workflow-job:version', '2.5') }}">
   {%- include "jenkins/files/jobs/_common.xml" %}
   <definition class="org.jenkinsci.plugins.workflow.cps.CpsFlowDefinition" plugin="workflow-cps@{{ salt['grains.get']('jenkins_plugins:workflow-cps:version', '2.12') }}">
     <script>// libraries