fix(bean_format): fix bean_format for beancount v3

[stasjok]

bean-format from beancount v3 requires '-' argument to read from stdin.
beancount v2 works both ways.

$ bean-format --version
Beancount 3.1.0

$ bean-format <ledger.beancount
Usage: bean-format [OPTIONS] FILENAME
Try 'bean-format --help' for help.

Error: Missing argument 'FILENAME'.

With '-' both version works:

$ bean-format --version
Beancount 2.3.6
$ bean-format - <ledger.beancount
;; -*- mode: org; mode: beancount; -*-
...

$ bean-format --version
Beancount 3.1.0
$ bean-format - <ledger.beancount
;; -*- mode: org; mode: beancount; -*-
...