sample: fix CustomScrollable sample to prevent too high Upper values

Upper values having higher values than PageSize or PageIncrement cause
rendering issues. This commit is a small adaptation of a fix from one
of the edge cases pointed out in this commit in hyena:
https://git.gnome.org/browse/hyena/commit/?h=gtk3&id=0745bfb75809886925dfa49a57c79e5f71565d08