Fix very unintuitive usage description

[Shumerez]

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

[Shumerez]

Also in RCPT mode section "Troubleshooting EXPN enumeration" named, well, as "Troubleshooting EXPN enumeration"
It should be "Troubleshooting RCPT enumeration", I guess