Use more PEP 570 syntax in the documentation. (GH-13720)

diff --git a/Doc/library/functools.rst b/Doc/library/functools.rst
index 654a3ef..3a0b554 100644
--- a/Doc/library/functools.rst
+++ b/Doc/library/functools.rst
@@ -252,7 +252,7 @@
       18
 
 
-.. class:: partialmethod(func, *args, **keywords)
+.. class:: partialmethod(func, /, *args, **keywords)
 
    Return a new :class:`partialmethod` descriptor which behaves
    like :class:`partial` except that it is designed to be used as a method