Fix syntax error in abn-tree.yml file (#4093)

There were 2 issues:

- YAML is indentation-sensitive, so descendants need indentation
- You can't mix object keys with array elements eg:

```yml
- one:
    title: Title
    - Invalid
```
So if you want to nest, then you can't use the shorthand:
```yml
- file/folder: Pretty name
```
You have to use:
```
- file/folder:
    title: Pretty name
    children:
        - document.md: A document
```

docs/abn-tree.yml

@@ -1,5 +1,6 @@
 - tutorials:
     title: Tutorials
+    children:
         - creating-your-first-adf-application.md: 'Creating your first ADF application'
         - creating-the-app-using-yeoman.md: 'Creating your ADF application using Yeoman'
         - creating-javascript-app-using-alfresco-js-api.md: 'Creating your JavaScript application using alfresco-js-api'