Last round of adapting style of documenting argument default values.
diff --git a/Doc/library/tkinter.tix.rst b/Doc/library/tkinter.tix.rst
index 7cd343c..6298bac 100644
--- a/Doc/library/tkinter.tix.rst
+++ b/Doc/library/tkinter.tix.rst
@@ -45,7 +45,7 @@
 ---------
 
 
-.. class:: Tix(screenName[, baseName[, className]])
+.. class:: Tk(screenName=None, baseName=None, className='Tix')
 
    Toplevel widget of Tix which represents mostly the main window of an
    application. It has an associated Tcl interpreter.