docs(ADVANCED.md): Update switch between rg/fzf example with new feature

[LangLangBart]

description

Make use of the transform action and the $FZF_PROMPT environment variable to
toggle between ripgrep and fzf with a single hotkey.

[junegunn]

Thanks.

The use of transform action is obviously more "advanced" and may be a little daunting for some readers. So how about keeping the current example, and adding the new one with transform after it as a separate section?

[LangLangBart]

I understand. I will make the necessary adjustments.

[junegunn]

Love it, thanks!