Singular form just like the other error message.

Modules/_pickle.c

@@ -5954,7 +5954,7 @@ Unpickler_set_memo(UnpicklerObject *self, PyObject *obj)
                 goto error;
             if (idx < 0) {
                 PyErr_SetString(PyExc_ValueError,
-                                "memos key must be positive integers.");
+                                "memo key must be positive integers.");
                 goto error;
             }
             if (_Unpickler_MemoPut(self, idx, value) < 0)