docs: fix path to openvz network configuration file

It's vznet.conf not vznetctl.conf, see e.g.:

http://git.openvz.org/?p=vzctl;a=blob;f=bin/vznetcfg.in;h=e91f5c4a0744c1ea149e1b8c241b666052e10b12;hb=HEAD

docs/drvopenvz.html.in

@@ -67,7 +67,7 @@ openvz+ssh://root@example.com/system (remote access, SSH tunnelled)
     script must be created manually by the host OS administrator. The
     simplest way is to just download the latest version of this script
     from a newer OpenVZ release, or upstream source repository. Then
-    a generic configuration file <code>/etc/vz/vznetctl.conf</code>
+    a generic configuration file <code>/etc/vz/vznet.conf</code>
     must be created containing
     </p>