man: use ~/.config instead of XDG_CONFIG_HOME

man: use ~/.config instead of XDG_CONFIG_HOME

e3820eeaf11f3b4614cbdfbc85675bc16a486e21 did that replacement XDG_CONFIG_HOME, in one
of two places. Let's use ~/.config everywhere.

Quoting https://github.com/systemd/systemd/pull/18704#discussion_r579465254:
> I'd really drop XDG_CONFIG_HOME from the docs. It's confusing enough as it
> is. Where we don't need the indirections we should not confuse people with
> it, in particular as people might then think it's actually a good idea to use
> that env var and redirect things. I'd just show the literal path everywhere,
> even if we internally use the env var.