r/nextcloud: Correct memcache config syntax

Nextcloud uses double backslashes in its fully-qualified path names.
Although single backslashes work, the application will replace them,
leading to a constant conflict between itself and the Ansible template.

roles/nextcloud/templates/config.php.j2

@@ -58,8 +58,8 @@ $CONFIG = array (
   'mail_smtpport' => '{{ nextcloud_smtp.port }}',
 {% endif %}
   'filelocking.enabled' => true,
-  'memcache.distributed' => '\OC\Memcache\Redis',
-  'memcache.locking' => '\OC\Memcache\Redis',
+  'memcache.distributed' => '\\OC\\Memcache\\Redis',
+  'memcache.locking' => '\\OC\\Memcache\\Redis',
   'redis' =>
   array  (
     'host' => 'localhost',