Revert "fix an error in readme"

This reverts commit 86e1089.

See also:
#46 (comment)

I will be recorrecting the typo myself immediately after this ref, and
licensing it MIT, to complete our licensing conversion.

Author: tony

README.rst

@@ -98,7 +98,7 @@ grab remaining tmux window:
 
 .. code-block:: python
 
-    >>> window = session.attached_window
+    >>> window = session.attached_window()
     >>> window.split_window(attach=False)
     Pane(%23 Window(@10 1:bar, Session($3 foo)))