juju config cmd help message is wrong for --file option

[Joe Guo]

In `juju config --help`:

```
apache2:
  servername: "example.com"
  lb_balancer_timeout: 60

If the above yaml document is stored in a file called config.yaml, the
following command can be used to apply the config changes:

juju config --file config.yaml
```

I got error for that cmd: "ERROR no application name specified"

Actually I have to specify app name:

juju config apache2 --file config.yaml

This doesn't make sense:

1) app name is already in config.yaml, no need to repeat.
2) I may config multiple apps in config.yaml, but with this name required, I can not do it.

juju version: 2.8.3-focal-amd64

[Facundo Ciccioli (fandanbango)]

I can confirm this bug's still present in the current develop branch (commit 652e1bcc).