Fix cmd_nessus_server_properties help text

cmd_nessus_server_properties help text was incorrect and referred
to nessus_server_feed. Fix the help text to correctly reflect
command name.

Thanks @void-in

plugins/nessus.rb

@@ -427,8 +427,8 @@ module Msf
         while (arg = args.shift)
           case arg
           when '-h', '--help'
-            print_status("nessus_server_feed")
-            print_status("Example:> nessus_server_feed -S searchterm")
+            print_status("nessus_server_properties")
+            print_status("Example:> nessus_server_properties -S searchterm")
             print_status()
             print_status("Returns information about the feed type and server version.")
             return