Fix doc link

doc/usage.md

@@ -5,7 +5,7 @@ Installation
 ------------
 
 Monolog is available on Packagist ([monolog/monolog](http://packagist.org/packages/monolog/monolog))
-and as such installable via [Composer](getcomposer.org/).
+and as such installable via [Composer](http://getcomposer.org/).
 
 If you do not use Composer, you can grab the code from GitHub, and use any
 PSR-0 compatible autoloader (e.g. the [Symfony2 ClassLoader component](https://github.com/symfony/ClassLoader))