Update backup.rst

Mention `custom_apps` directory.

Signed-off-by: Ricky Sixx <51907568+rickysixx@users.noreply.github.com>

Author: rickysixx

admin_manual/maintenance/backup.rst

@@ -2,9 +2,10 @@
 Backup
 ======
 
-To backup a Nextcloud installation there are four main things you need to retain:
+To backup a Nextcloud installation there are five main things you need to retain:
 
 #. The config folder
+#. The custom apps folder (only if you have custom apps installed)
 #. The data folder
 #. The theme folder
 #. The database