Fix very unintuitive usage description

`usage: smtp-user-enum [options] -u/-U host port`

Is red as `smtp-user-enum example.com 25`
But in fact it is `smtp-user-enum -u admin example.com 25`
Or `smtp-user-enum -U usernames.txt example.com 25`

Tried ~8 times, then just found answer in sources. It is subtle and actually not as in other popular tools. I suggest to change description to
`usage: smtp-user-enum [options] -u username/-U list_of_usernames.txt host port`
or smth alike

Pls change description