[cloud][docs] update ec2_tag documentation example (#22586)

Currently the documentation utilizes with_subelements, but it does not parse the results correctly.  By changing to with_items: and specifying the proper list, we are able to tag the instances as expected.

lib/ansible/modules/cloud/amazon/ec2_tag.py

@@ -100,9 +100,8 @@ EXAMPLES = '''
     tags:
       Name: dbserver
       Env: production
-  with_subelements:
+  with_items:
-    - ec2_vol.results
+    - ec2_vol.volumes
-    - volumes
 
 - name: Get EC2 facts
   action: ec2_facts