Fix the unstable_test syntax for test_live_migration_with_trunk

test_live_migration_with_trunk test was marked
as unstable in
- https://review.opendev.org/c/openstack/tempest/+/895167

but it used the wrong syntax for unstable_test
decorator.

Change-Id: I8f4f3cafe8265984470bf45a9d85b3c9f765a251
1 file changed