bpo-35770: Fix off-by-1 error. (#11618)

diff --git a/Lib/idlelib/macosx.py b/Lib/idlelib/macosx.py
index d6a1b37..eeaab59 100644
--- a/Lib/idlelib/macosx.py
+++ b/Lib/idlelib/macosx.py
@@ -178,7 +178,7 @@
     del mainmenu.menudefs[-1][1][0:2]
     # Remove the 'Configure Idle' entry from the options menu, it is in the
     # application menu as 'Preferences'
-    del mainmenu.menudefs[-3][1][0:1]
+    del mainmenu.menudefs[-3][1][0:2]
     menubar = Menu(root)
     root.configure(menu=menubar)
     menudict = {}