introspection: use correct node name for ModemManager1 object
In the D-Bus specification it is stated that the root node of an introspection file should have an absolute path[1]. This path should not be the root path in D-Bus as putting objects there is considered incorrect, and also because the MM1 object does not actually sit at the root path (instead, it uses MM_DBUS_PATH, which is /org/freedesktop/ModemManager1). Fix this in the introspection file.
[1] https://dbus.freedesktop.org/doc/dbus-specification.html#introspection-format