bpo-37116: Use PEP 570 syntax for positional-only parameters. (GH-13700)

diff --git a/Doc/library/contextlib.rst b/Doc/library/contextlib.rst
index 017a87a..73b24e5 100644
--- a/Doc/library/contextlib.rst
+++ b/Doc/library/contextlib.rst
@@ -637,7 +637,7 @@
    from contextlib import ExitStack
 
    class Callback(ExitStack):
-       def __init__(self, callback, *args, **kwds):
+       def __init__(self, callback, /, *args, **kwds):
            super(Callback, self).__init__()
            self.callback(callback, *args, **kwds)