shortcuts-window: Update the entry for Page Down / Page Up

Reverse the order of buttons so that it's clear that Page Down is
supposed to seek to the previous chapter and Page Up is supposed to seek
to the next chapter.

Fixes #968.

src/celluloid-shortcuts-window.c

@@ -62,7 +62,7 @@ celluloid_shortcuts_window_init(CelluloidShortcutsWindow *wnd)
 			{"<Shift>downarrow <Shift>uparrow", _("Exact seek backward/forward 5 seconds")},
 			{"<Ctrl>Left <Ctrl>Right", _("Seek to previous/next subtitle")},
 			{"comma period", _("Step backward/forward a single frame")},
-			{"Page_Up Page_Down", _("Seek to the beginning of the previous/next chapter")},
+			{"Page_Down Page_Up", _("Seek to the beginning of the previous/next chapter")},
 			{NULL, NULL} };
 	const ShortcutEntry playback[]
 		= {	{"bracketleft bracketright", _("Decrease/increase playback speed by 10%")},