Update broken link in CONTRIBUTING.md file

Author: divineniiquaye

.github/CONTRIBUTING.md

@@ -4,7 +4,7 @@ Contributions are **welcome** and will be fully **credited**. We accept contribu
 
 ## Pull Requests
 
-- **[PSR-12 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-12-coding-style-guide.md)** - The easiest way to apply the conventions is to install [PHP Code Sniffer](http://pear.php.net/package/PHP_CodeSniffer).
+- **[PSR-12 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-12-extended-coding-style-guide.md)** - The easiest way to apply the conventions is to install [PHP Code Sniffer](http://pear.php.net/package/PHP_CodeSniffer).
 
 - **Add tests!** - Your patch won't be accepted if it doesn't have tests.