Update rabbitmq.rst

Example on how to use RabbitMQ with custom vhost and credentials.
"UseRabbitMqRouting()" is deprecated according to https://groups.google.com/d/msg/masstransit-discuss/4m5Vf04oRWM/Z2veL6IcaGEJ so removing all references to it.
Update rabbitmq.rst

Fixed comments from @TravisTheTechie