[chore] add to docs the default value of the html class option.
[chore] add to docs the default value of the html class option.

rackt/react-modal,diasbruno/react-modal,bluSCALE4/react-modal,diasbruno/react-modal,reactjs/react-modal,springload/react-modal,bluSCALE4/react-modal,springload/react-modal,rackt/react-modal,diasbruno/react-modal,reactjs/react-modal,bluSCALE4/react-modal,reactjs/react-modal,springload/react-modal