Fix links to primer website

stylesheets/README.md

@@ -5,8 +5,7 @@ This website uses a Sass preprocessor, and gets most of its styles from GitHub's
 
 All styles come from imports in our Next.js compnents (pages, components).
 
-In general, we use Primer's [utility classes](https://styleguide.github.com/primer/utilities/)
-as much as we can, and avoid writing custom styles whenever possible.
+In general, we use Primer's [utility classes](https://primer.style/css/utilities) as much as we can, and avoid writing custom styles whenever possible.
 
-See [styleguide.github.com/primer](https://styleguide.github.com/primer/) for reference.
+See [https://primer.style/](https://primer.style/) for reference.