apprt/gtk: fix typo

I had a copy-paste error when I used right instead of up.

Signed-off-by: Tristan Partin <tristan@partin.io>

src/apprt/gtk/Window.zig

@@ -1058,7 +1058,7 @@ fn gtkActionSplitUp(
     _: ?*glib.Variant,
     self: *Window,
 ) callconv(.C) void {
-    self.performBindingAction(.{ .new_split = .right });
+    self.performBindingAction(.{ .new_split = .up });
 }
 
 fn gtkActionToggleInspector(