Fix typo error in listing server's column name

openstack server list -c "Created At" command doesn't work
because the wrong variable was used here.

When we receive resp data, Created At data is saved with
the name "created". But in "server.py", we append columns
as created_at. So it seems to print an empty table.

Story: 2009149
Task: 43112
Change-Id: I06de6903d5cc427a8b0fdcd168fec47192f4365b

openstackclient/compute/v2/server.py

@@ -2341,7 +2341,7 @@ class ListServer(command.Lister):
                     columns.append('user_id')
                     column_headers.append('User ID')
                 if c in ('Created At', 'created_at'):
-                    columns.append('created_at')
+                    columns.append('created')
                     column_headers.append('Created At')
 
             # convert back to tuple