Corrected the API endpoints used for DKIM and return path verification

[indigogiant]

It seems that the API endpoints used the VerifyDomainDkim and VerifyDomainReturnPath are switched around. This PR swaps them so that they're correct.

I notice that both endpoints seem to have identical functionality, but the concern here is whether there are differences in the current (switched) behaviour that's being relied upon by existing consumers of the client. While I've simply switched the endpoints here an alternate option would be to mark the existing methods [Obsolete] and introduce new methods with the corrected behaviour (perhaps adding the Async suffix too).