Updated concurrency example to cancel any in-progress job or run

changed `group: ${{ github.head_ref }}`  to ${{ github.head}}`

Author: jpbnetley

data/reusables/actions/actions-group-concurrency.md

@@ -19,7 +19,7 @@ concurrency: ci-${{ github.ref }}
 {% raw %}
 ```yaml
 concurrency: 
-  group: ${{ github.head_ref }}
+  group: ${{ github.ref }}
   cancel-in-progress: true
 ```
 {% endraw %}