In the documentation in the section "Integrating the transpile task" just before the section "Checking everything works" there is a small section that talks about updating the html task. The code change should be applied to the var optimizeHtmlTask and not be edited directly in the html task.
The gulpfile in the gh repo is ok and has the correct code in it.
The documentation should read:
var optimizeHtmlTask = function(src, dest) { var assets = $.useref.assets({ searchPath: ['.tmp', 'dist'] });