[docs] [issue39267] Fix dict's __missing__ documentation

Change by Bar Harel <bzvi7919@gmail.com>: ---------- keywords: +patch pull_requests: +17322 stage: -> patch review pull_request: https://github.com/python/cpython/pull/17911 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue39267> _______________________________________