Incorrect example for Gtk.ScaleButton

[fnevgeny]

Hello,

In the sample code at https://valadoc.org/gtk+-3.0/Gtk.ScaleButton.html, please change

string[] icons = { "zoom-out", "zoom-in" };

to

const string[] icons = { "zoom-out", "zoom-in", null };

"icons" should be a null-terminated array.