snapcraft close example recommends non-existant flag

[John A Meinel]

Running `snapcraft close help` it gives this example:
$ snapcraft close --help
Usage:
    snapcraft close [options] <name> <channel>
...
    Examples:
        snapcraft close my-snap --channel beta

However, it does not support a `--channel` flag, and the example is incorrect.

Following the example gives a failure:
$ snapcraft close juju --channel 2.4/stable
Usage: snapcraft [options] command [args]...
Try 'snapcraft close -h' for help.

Error: unrecognized arguments: --channel

$ snapcraft --version
snapcraft 7.0.7

(all it really needs is to remove the `--channel` from the example command)