look for octoprint as a octoprint service, not http.

Reference:
https://github.com/foosel/OctoPrint/wiki/Plugin:-Discovery#zeroconf-service-_octoprint_tcp

Fixes #4375

lib/Slic3r/GUI/PresetEditor.pm

@@ -1323,7 +1323,7 @@ sub build {
                 # look for devices
                 my $entries;
                 {
-                    my $res = Net::Bonjour->new('http');
+                    my $res = Net::Bonjour->new('octoprint');
                     $res->discover;
                     $entries = [ $res->entries ];
                 }