The doc string for 'format' contains

  where field is [0-9]+ followed by a literal dollar "$", flags is
  [+ #-0]+, width is [0-9]+, and precision is a literal period "."
  followed by [0-9]+.

The spec for flags isn't a correct regexp; the hyphen should be last.  This matters, because people copy it into their code without thinking (observed in the wild).

Patch acceptable for emacs-27?