firstboot: Make the option list fit 80 columns

22 characters in three colums + overhead slightly exceeds the available
width on terminals with 80 columns, causing each row to wrap to two lines.
Reduce the item width to 20 to fit even the list of ~600 timezones.

src/firstboot/firstboot.c

@@ -215,7 +215,7 @@ static int prompt_loop(const char *text, char **l, unsigned percentage, bool (*i
                 }
 
                 if (streq(p, "list")) {
-                        r = show_menu(l, 3, 22, percentage);
+                        r = show_menu(l, 3, 20, percentage);
                         if (r < 0)
                                 return r;