bpo-44273: Improve syntax error message for assigning to "..." (GH-26477)

Use "ellipsis" instead of "Ellipsis" in syntax error messages to eliminate confusion with built-in variable Ellipsis.

Parser/pegen.c

@@ -217,7 +217,7 @@ _PyPegen_get_expr_name(expr_ty e)
                 return "True";
             }
             if (value == Py_Ellipsis) {
-                return "Ellipsis";
+                return "ellipsis";
             }
             return "literal";
         }