Fix a typo.

Author: serhiy-storchaka

Lib/test/test_tkinter/test_misc.py

@@ -558,7 +558,7 @@ def test_wm_iconbitmap(self):
         t = tkinter.Toplevel(self.root)
         self.assertEqual(t.wm_iconbitmap(), '')
         t.wm_iconbitmap('hourglass')
-        if root._windowingsystem != 'aqua':  # Bug in Tk.
+        if t._windowingsystem != 'aqua':  # Bug in Tk.
             self.assertEqual(t.wm_iconbitmap(), 'hourglass')
         self.assertEqual(self.root.wm_iconbitmap(), '')
         t.wm_iconbitmap('')