Gtk3: Fix wrong filename of scrollbar

`(gtk3-demo:5797): Gtk-WARNING **: Error loading image
'file:///usr/share/themes/Breeze/gtk-3.0/assets/scrollbar-slider-
overlay-horizontal.png': Error opening file: No such file or directory`

the image file in assets name `scrollbar-slider-horizontal-overlay.png`
but in css file was `scrollbar-slider-overlay-horizontal.png`.

REVIEW: 127323
REVIEW: 126970
BUG: 356395