bpo-33924: Change IDLE mainmenu.menudefs key 'windows' to 'window' (GH-7836)

Every other menudef key is the lowercase version of the
corresponding main menu entry (in this case, 'Window').
diff --git a/Lib/idlelib/macosx.py b/Lib/idlelib/macosx.py
index 3d29612..a2260af 100644
--- a/Lib/idlelib/macosx.py
+++ b/Lib/idlelib/macosx.py
@@ -148,7 +148,7 @@
     root.configure(menu=menubar)
     menudict = {}
 
-    menudict['windows'] = menu = Menu(menubar, name='windows', tearoff=0)
+    menudict['window'] = menu = Menu(menubar, name='window', tearoff=0)
     menubar.add_cascade(label='Window', menu=menu, underline=0)
 
     def postwindowsmenu(menu=menu):