Remove reference to responseHTML

As part of the work on the TypeScript jQuery typings we believe we found an erroneous reference to ```responseHTML``` in the API docs.  This PR removes reference to ```responseHTML``` in favour of ```responseXML```.  You can read details of the rationale here:

https://github.com/borisyankov/DefinitelyTyped/pull/1471#issuecomment-31204115