man: use ~/.config instead of XDG_CONFIG_HOME

e3820eeaf1 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.

man/systemd.unit.xml

@@ -60,7 +60,7 @@
 <filename>$XDG_RUNTIME_DIR/systemd/user.control/*</filename>
 <filename>$XDG_RUNTIME_DIR/systemd/transient/*</filename>
 <filename>$XDG_RUNTIME_DIR/systemd/generator.early/*</filename>
-<filename>$XDG_CONFIG_HOME/systemd/user/*</filename>
+<filename>~/.config/systemd/user/*</filename>
 <filename>$XDG_CONFIG_DIRS/systemd/user/*</filename>
 <filename>/etc/systemd/user/*</filename>
 <filename>$XDG_RUNTIME_DIR/systemd/user/*</filename>