Fix a typo in the help message printed by vhost-user-bridge.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210901152713.25701-1-peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>