Update Supported GTK4 Distro-list

[ghost]

This addition adds Ubuntu and Arch Linux to the supported GTK4 distros.

[ToshioCP]

Hello, Shulkk.

Thank you for the pull request. I've merged your request, but I'd like to write some comments.

• Your update in gfm/sec16.md and gfm/sec9.md are the output of 'wc' and 'glib-compile_resources' respectively. They are generated automatically when a developer runs rake. So, they depend on the developer's environment and they will be changed again if I run rake in the future.

• Your update in gfm/sec2.src.md will remain even if I run rake. I was so lazy that the section 2 hasn't updated untill now. Gtk4 has developed more since I wrote the section. I'll modify it as soon as possible.

Anyway, thank you again for your help. I'll be happy if you continue to help me.

Regards,

Toshio