Update rbs_worldpay.rst

Looks like someone has created a special case world_pay template tag and not updated the docs?
I'm using this in my project now and it's working however I'm not sure if the settings are working properly. I had to:

world_pay = get_integration("world_pay") 
worldpay.add_fields(settings.MERCHANT_SETTINGS['world_pay'])

Will hopefully get around to having a look at soon.