Исправляет ссылку на Primer Guidelines от GitHub (htmlacademy#71)

Author: staticinside

Readme.md

@@ -7,7 +7,7 @@
 * [Code Guide](http://codeguide.co) by @mdo
 * [CSS Guidelines](http://cssguidelin.es) by Harry Roberts
 * [Idiomatic CSS](https://github.com/necolas/idiomatic-css) by Nicolas Gallagher
-* [Primer Guidelines](https://styleguide.github.com/primer/principles/) by GitHub
+* [Primer Guidelines](https://primer.style/css/) by GitHub
 
 ## Лицензия