Normalize spelling of Mac OS X.
diff --git a/Doc/library/ttk.rst b/Doc/library/ttk.rst
index f38664e..1795771 100644
--- a/Doc/library/ttk.rst
+++ b/Doc/library/ttk.rst
@@ -656,7 +656,7 @@
 Platform-specific notes
 ^^^^^^^^^^^^^^^^^^^^^^^
 
-* On MacOS X, toplevel windows automatically include a built-in size grip
+* On Mac OS X, toplevel windows automatically include a built-in size grip
   by default. Adding a :class:`Sizegrip` is harmless, since the built-in
   grip will just mask the widget.