plugin_pack.py: another python3 compat fix in show_usage
More specifically, fixing something that was broken in a previous python3
compat attempt by someone else.
[Chat User List Logging]
type=default
depends=pidgin
provides=listlog
summary=Logs the list of users in a chat when you join
description=When a chat is joined, this plugin will print the list of users in the chat in the conversation window, thus causing the userlist to be logged