Add appstoreurl to config.sample.php.

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>

config/config.sample.php

@@ -913,6 +913,14 @@ $CONFIG = [
  */
 'appstoreenabled' => true,
 
+/**
+ * Enables the installation of apps from a self hosted apps store.
+ * Requires that at least one of the configured apps directories is writeable.
+ *
+ * Defaults to ``https://apps.nextcloud.com/api/v1``
+ */
+'appstoreurl' => 'https://apps.nextcloud.com/api/v1',
+
 /**
  * Use the ``apps_paths`` parameter to set the location of the Apps directory,
  * which should be scanned for available apps, and where user-specific apps