shells/pbosh: Update to 20210607

Changelog from AN-2021-05-19:
- Bourne Shell: the fc(1) builtin now handles "fc -l -0" the same
  way as ksh does and refers to the current command. Before only
  "fc -l -1" did work to select the previous command.

  Together with the "ERR" trap fix, this permits to install a
  "command not found" handler in the shell that could explain users
  how to install a missing command.

  This "missing command" handler could be implemented via:

    trap 'show-info-message-for-missing-command $(fc -l -0)' ERR

Changelog from AN-2021-06-07:
- Bourne Shell: the man page now uses a correct notation for "[]" for
  the arguments of the builtin "dosh" that has been introduced in May
  2012.

(micha)