Update example to work (#77191)

`with_first_found` only takes a list of things, it's not like `tags`
where it can take a thing or a list of things.

Inspired by locally discovered breakage and then finding
https://github.com/ansible/ansible/issues/77136 where someone was also
doing this and getting into trouble.

lib/ansible/plugins/lookup/first_found.py

@@ -67,7 +67,7 @@ EXAMPLES = """
   ansible.builtin.include_tasks:
     file: "{{ item }}"
   with_first_found:
-    files:
+    - files:
       - path/tasks.yaml
       - path/other_tasks.yaml
       skip: True