Fix property name of viewport width/height settings in Multiple resolutions (#811)

Co-authored-by: Hugo Locurcio <[email protected]>

gui/multiple_resolutions/main.gd

@@ -4,7 +4,10 @@
 # (with their rect spread across the whole viewport, and Anchor set to Full Rect).
 extends Control
 
-var base_window_size = Vector2(ProjectSettings.get_setting("display/window/size/width"), ProjectSettings.get_setting("display/window/size/height"))
+var base_window_size = Vector2(
+		ProjectSettings.get_setting("display/window/size/viewport_width"),
+		ProjectSettings.get_setting("display/window/size/viewport_height")
+)
 
 # These defaults match this demo's project settings. Adjust as needed if adapting this
 # in your own project.