Law Roach says working with Celine Dion helped him get better at his job.Dia Dipasupil/Getty ImagesLaw Roach says working with Celine Dion challenged him to be a better stylist.At their first fitting, the singer pointed out every adjustment a gown needed before he could.The singer also taught him that slowing down is the fastest way to get things right.Celebrity stylist Law Roach says working with Celine Dion had a lasting impact on his career."…