Use the zero argument form of super() in examples for Python3 docs. (GH-22314)

diff --git a/Doc/library/contextlib.rst b/Doc/library/contextlib.rst
index 91edbba..b92f703 100644
--- a/Doc/library/contextlib.rst
+++ b/Doc/library/contextlib.rst
@@ -753,7 +753,7 @@
 
    class Callback(ExitStack):
        def __init__(self, callback, /, *args, **kwds):
-           super(Callback, self).__init__()
+           super().__init__()
            self.callback(callback, *args, **kwds)
 
        def cancel(self):