doc: fix wrong ssh-keyscan usage

[oluceps]

ssh-keyscan seems doesn't accept the format user@address.
See https://man.openbsd.org/ssh-keyscan.1

[cole-h]

Confirmed:

$ ssh-keyscan root@localhost
getaddrinfo root@localhost: Name or service not known
^C                                                        

$ ssh-keyscan localhost
# localhost:22 SSH-2.0-OpenSSH_9.7
[..snip..]