networkAgent.js: Fix plugin property lookup

The section in nm-openvpn-service.name, provided by
network-manager-openvpn, is specifically called GNOME.

[GNOME]
auth-dialog=/usr/libexec/nm-openvpn-auth-dialog
properties=libnm-openvpn-properties
supports-external-ui-mode=true
supports-hints=true

This commit is necessary for the Clutter agent to show up.

Note also that the GTK agent won't show up if nm-applet is blacklisted
(by cinnamon-session).

Author: clefebvre

js/ui/networkAgent.js

@@ -764,7 +764,7 @@ var NetworkAgent = class {
             return null;
         }
 
-        const prop = plugin.lookup_property('CINNAMON', 'supports-external-ui-mode');
+        const prop = plugin.lookup_property('GNOME', 'supports-external-ui-mode');
         const trimmedProp = prop ? prop.trim().toLowerCase() : '';
 
         return {