RainerScript chapter rename to advanced configuration format

[Zugschlus]

trafficstars

Hi,

as mentioned in https://www.rsyslog.com/doc/v8-stable/configuration/conf_formats.html, the thing that was called RainerScript previously is now called advanced configuration format.

This should be reflected in the chapter title.

This is too big a change that I dare filing a PR because I don't know enough about sphinx to judge whether there is a need to rename cross-references in the other chapters. Sorry.

Greetings Marc