[docs] [issue13790] In str.format an incorrect error message for list, tuple, dict, set

Change by Terry J. Reedy <tjreedy@udel.edu>: ---------- pull_requests: +18049 pull_request: https://github.com/python/cpython/pull/18690 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue13790> _______________________________________