Correct reference

.taskcluster.yml

@@ -137,6 +137,6 @@ tasks:
                     --commit-range ${event.pull_request.base.sha}...
                     ${browser.name}
                     --
-                    ${command.extra_args} || result=$?;
+                    ${operation.extra_args} || result=$?;
                   echo $result > ../artifacts/run-return-code.txt;
                   echo Command exited with code $result (failures are allowed while this task is being vetted).