Improve mapDispatchToProps factory function documentation

[fongandrew]

Some examples were added for factory functions on https://github.com/reactjs/react-redux/blob/master/docs/api.md per #797, but the example for mapDispatchToPropsFactory seems off.

Here's the example:

function mapDispatchToPropsFactory(initialState, initialProps) {
  function goToSomeLink(){
    initialProps.history.push('some/link');
  }
  return function(dispatch){
    return {
      addTodo
    }
  }
}

The function defines goToSomeLink but it's not referenced in any of the return values.

[fongandrew]

@markerikson

[timdorr]

Yeah, wait...wut. I guess I merged that too soon. I'm not a factory user, so my eyes tend to glaze over at these things. Whoops!