Update contributing guide to list PER-CS.

CONTRIBUTING.md

@@ -7,7 +7,7 @@ We accept contributions via Pull Requests on [Github](https://github.com/Crell/T
 
 ## Pull Requests
 
-- **[PSR-2 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)** - Check the code style with ``$ composer check-style`` and fix it with ``$ composer fix-style``.
+- **[PER-CS Coding Standard](https://www.php-fig.org/per/coding-style/)** - Check the code style with ``$ composer check-style`` and fix it with ``$ composer fix-style``.
 
 - **Add tests!** - Your patch won't be accepted if it doesn't have tests.