Update Marketo spec with correct expectation

With the recent changes in 18c4c6a, it actually breaks with the WSDL I'm using so I thought I'd run the tests to confirm and I noticed that the spec was expecting the incorrect message tag. Not sure if this _needs_ to be merged in considering it appears that the WSDL I'm using may not be standards compliant.

Examples can be seen [here](http://developers.marketo.com/documentation/soap/getlead/).