Adjust GDscript example formatting in Using Containers (#5020)

tutorials/gui/gui_containers.rst

@@ -170,7 +170,7 @@ to its rect size:
     extends Container
 
     func _notification(what):
-        if (what==NOTIFICATION_SORT_CHILDREN):
+        if what == NOTIFICATION_SORT_CHILDREN:
             # Must re-sort the children
             for c in get_children():
                 # Fit to own size