bpo-39973: Fix the docs for PyObject_GenericSetDict() (GH-19026)

PyObject_GenericSetDict() takes three arguments, not two.
diff --git a/Doc/c-api/object.rst b/Doc/c-api/object.rst
index 2905fbb..b9c137e 100644
--- a/Doc/c-api/object.rst
+++ b/Doc/c-api/object.rst
@@ -128,7 +128,7 @@
    .. versionadded:: 3.3
 
 
-.. c:function:: int PyObject_GenericSetDict(PyObject *o, void *context)
+.. c:function:: int PyObject_GenericSetDict(PyObject *o, PyObject *value, void *context)
 
    A generic implementation for the setter of a ``__dict__`` descriptor. This
    implementation does not allow the dictionary to be deleted.