Fix numbering in custom installers article

doc/articles/custom-installers.md

@@ -56,7 +56,7 @@ A basic Installer Plugin would thus compose of three files:
 
 1. the package file: composer.json
 2. The Plugin class, e.g.: `My\Project\Composer\Plugin.php`, containing a class that implements `Composer\Plugin\PluginInterface`.
-2. The Installer class, e.g.: `My\Project\Composer\Installer.php`, containing a class that implements `Composer\Installer\InstallerInterface`.
+3. The Installer class, e.g.: `My\Project\Composer\Installer.php`, containing a class that implements `Composer\Installer\InstallerInterface`.
 
 ### composer.json