Fix for issue3559: No preferences menu in IDLE on OSX

1) Add a comment to the help file to that points to the
   preferences menu.

2) An earlier checkin tried to detect Tk >= 8.10.14,
   but did this in the wrong way. The end result of this
   was that the IDLE->Preferences... menu got surpressed
   when using the system version of Tcl/Tk
diff --git a/Lib/idlelib/help.txt b/Lib/idlelib/help.txt
index 76cccf0..7bfd2ca 100644
--- a/Lib/idlelib/help.txt
+++ b/Lib/idlelib/help.txt
@@ -90,7 +90,10 @@
 	Configure IDLE -- Open a configuration dialog.  Fonts, indentation,
                           keybindings, and color themes may be altered.
                           Startup Preferences may be set, and Additional Help
-                          Souces can be specified.
+                          Sources can be specified.
+			  
+			  On MacOS X this menu is not present, use
+			  menu 'IDLE -> Preferences...' instead.
 	---
 	Code Context --	  Open a pane at the top of the edit window which
 			  shows the block context of the section of code