Apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: matthiasgoergens

Modules/_collectionsmodule.c

@@ -1833,7 +1833,7 @@ static PyMethodDef deque_methods[] = {
     DEQUE_ROTATE_METHODDEF
     DEQUE___SIZEOF___METHODDEF
     {"__class_getitem__",       Py_GenericAlias,
-        METH_O|METH_CLASS,       PyDoc_STR("See PEP <placeholder>")},
+        METH_O|METH_CLASS,       PyDoc_STR("See PEP 585")},
     {NULL,              NULL}   /* sentinel */
 };