scrolledwindow: fix copy/paste typo
scrolledwindow: fix copy/paste typo

fc2830394895f236352bea2024dda0e962dd2c53 refactored code into the
get_scroll_unit() function but introduced a copy/paste typo.

chergert/gtk,davidgumberg/gtk,alexlarsson/gtk,jigpu/gtk,chergert/gtk,chergert/gtk,grubersjoe/adwaita,jadahl/gtk,Adamovskiy/gtk,jadahl/gtk,jigpu/gtk,grubersjoe/adwaita,davidgumberg/gtk,davidgumberg/gtk,grubersjoe/adwaita,jadahl/gtk,Adamovskiy/gtk,jadahl/gtk,davidgumberg/gtk,jigpu/gtk,jigpu/gtk,Adamovskiy/gtk,Adamovskiy/gtk,alexlarsson/gtk,Adamovskiy/gtk,chergert/gtk,alexlarsson/gtk,jigpu/gtk,jigpu/gtk,Adamovskiy/gtk,jigpu/gtk,Adamovskiy/gtk,jadahl/gtk,grubersjoe/adwaita,jadahl/gtk,alexlarsson/gtk,alexlarsson/gtk,chergert/gtk,alexlarsson/gtk,alexlarsson/gtk,jadahl/gtk,alexlarsson/gtk,grubersjoe/adwaita,jadahl/gtk,chergert/gtk,grubersjoe/adwaita,chergert/gtk,Adamovskiy/gtk,davidgumberg/gtk,davidgumberg/gtk,davidgumberg/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jigpu/gtk,chergert/gtk,davidgumberg/gtk