Fix documentation
Fix documentation

Remove reference to AMQP messaging action as it doesn't exist.

Change-Id: I84801ec5f25857c1537b6e0688e2c28c345da3ca
Signed-off-by: Márton Csuha <2e9b9b53b3eaa73172232cbe16d1abbc1865ab3a@nokia.com>

openstack/mistral,StackStorm/mistral,StackStorm/mistral,openstack/mistral