README.md Document error, miss 'build' task

[Roamin]

What were you expecting to happen?
There is a build task
What actually happened?
Cann’t find it. (The sample written in ES2015)

What version of gulp are you using?
4.0
What versions of npm and node are you using?
It does'nt matter

[demurgos]

Instead of:

const clean = gulp.series(clean, gulp.parallel(styles, scripts));
gulp.task('clean', clean);

It should be:

const build = gulp.series(clean, gulp.parallel(styles, scripts));
gulp.task('build', build);

Thanks for reporting the issue, I'll send a PR.