Fix broken link in contributing guide

Author: carlbennett

CONTRIBUTING.md

@@ -97,7 +97,7 @@ Configure nginx so that it is a fastcgi proxy to php-fpm. You will then run
 
 Nginx should be pointed at the [src/main.php](./src/main.php) file for all
 requests. A sample config exists at
-[etc/nginx-vhost-sample.conf](./etc/nginx-vhost.sample.conf).
+[etc/nginx-vhost-sample.conf](./etc/nginx-vhost-sample.conf).
 
 MariaDB should be configured with the `TRADITIONAL,NO_AUTO_VALUE_ON_ZERO` modes.
 A sample configuration can be found at