I *think* that one is a setting in ProFTPD itself GeckoAndrew, assuming you've already run reconfigurator to correct the default server info. (If you haven't done that yet search the boards for something like reconfigurator and you should find it. Alien made an excellent post showing step-by-step how to fix that issue.)
For the ProFTPD part of it, I'd suggest taking a little trip by the ProFTPD website as they have a pretty good section on changing settings in the default FTP client.
http://www.proftpd.org is the addy, then kind of look around for the configuration examples. I don't remember where I found them exactly.
I've tweaked several things in my ProFTPD just because I hate having the server log me out when I'm idle for 2 minutes.

But the Server Name is also one of the settings I changed while I was there.
On the standard Plesk installations the file you'll need to edit can be found at "/usr/local/psa/ftpd/etc" and is called "proftpd.conf".
If all you want to change is the server name people see when they FTP in, that one will be listed as ServerName up towards the top. Just change it to whatever you want.
FYI, since this isn't covered well on the ProFTPD site, since we run it as ServerType inted, you can freely ignore the directions when they tell you that you need to restart ProFTPD. That's only required when running as a Standalone. With the inetd setup changes will take effect as soon as you save that file and someone FTP's in again.
HTH
Randy