Add 'contrib' to Drush commandfile installer path (drupal-composer#525)

Author: greg-1-anderson

composer.json

@@ -70,7 +70,7 @@
             "web/modules/contrib/{$name}": ["type:drupal-module"],
             "web/profiles/contrib/{$name}": ["type:drupal-profile"],
             "web/themes/contrib/{$name}": ["type:drupal-theme"],
-            "drush/Commands/{$name}": ["type:drupal-drush"]
+            "drush/Commands/contrib/{$name}": ["type:drupal-drush"]
         },
         "drupal-scaffold": {
             "initial": {