correct param names in subscription http example

looks like it was copied from the pre_authorization example so was
using the "pre_authorization[foo]" params when it should be using
"subscription[foo]". also change the host to be gocardless.com from
sandbox.gocardless.com to make it consistent with other examples
Merge pull request #36 from leejo/lee/correct_subscription_get_example

correct param names in subscription http example