Fix background scan doc in config

The background scanner only processes entries with size < 0

Signed-off-by: Vincent Petry <vincent@nextcloud.com>

config/config.sample.php

@@ -1977,7 +1977,7 @@ $CONFIG = [
  *
  * By default, a background job runs every 10 minutes and execute a background
  * scan to sync filesystem and database. Only users with unscanned files
- * (size=0 in filecache) are included. Maximum 500 users per job.
+ * (size < 0 in filecache) are included. Maximum 500 users per job.
  *
  * Defaults to ``true``
  */