wallabag: fix comment with usage instructions

pkgs/servers/web-apps/wallabag/default.nix

@@ -4,8 +4,10 @@
 }:
 
 # Point the environment variable $WALLABAG_DATA to a data directory
-# that contains the folder `app/config` which must be a clone of
+# that contains the folder `app` which must be a clone of
 # wallabag's configuration files with your customized `parameters.yml`.
+# In practice you need to copy `${pkgs.wallabag}/app` and the
+# customizzed `parameters.yml` to $WALLABAG_DATA.
 # These need to be updated every package upgrade.
 #
 # After a package upgrade, empty the `var/cache` folder or unexpected