Fix documented signatures for C API functions. (GH-11236)

diff --git a/Doc/c-api/tuple.rst b/Doc/c-api/tuple.rst
index a66832c..20bf9f0 100644
--- a/Doc/c-api/tuple.rst
+++ b/Doc/c-api/tuple.rst
@@ -209,7 +209,7 @@
       This function "steals" a reference to *o*.
 
 
-.. c:function:: PyObject* PyStructSequence_SET_ITEM(PyObject *p, Py_ssize_t *pos, PyObject *o)
+.. c:function:: void PyStructSequence_SET_ITEM(PyObject *p, Py_ssize_t *pos, PyObject *o)
 
    Macro equivalent of :c:func:`PyStructSequence_SetItem`.