Fix error message

Author: janodvarko

lib/har-driver-actor.js

@@ -276,7 +276,7 @@ function ExportDriverApi(actor) {
  function securityCheck(method) {
     return function(options) {
       if (options.token != actor.token) {
-        let pref = "devtools.netmonitor.har.contentAPIToken";
+        let pref = "extensions.netmonitor.har.contentAPIToken";
         let msg = "Security check didn't pass. You need to set '" +
           pref + "' pref to match the string token passed into " +
           "HAR object API call (browser restart is required)";