Change the example of playbook include. (#22697)

This (taken from https://github.com/ansible/ansible-examples/blob/master/language_features/nested_playbooks.yml) is a better example. It makes it clear that the full file name is used, and a single filename, as opposed to a YAML list. The old version, it was not clear if 'list_of_plays' was an *example* or a *variable*.

docs/docsite/rst/playbooks_roles.rst

@@ -37,7 +37,7 @@ Task versus Play includes
 Tasks and plays both use the `include` keyword, but implement the keyword differently. The difference between them is determined by their positioning and content. If the include is inside a play it can only be a 'task' include and include a list of tasks; if it is at the top level, it can only include plays. For example::
 
     # this is a 'play' include
-    - include: listofplays
+    - include: intro_example.yml
 
     - name: another play
       hosts: all