nixos/bookstack: fix error message output (#142729)

nixos/modules/services/web-apps/bookstack.nix

···
 
     assertions = [
       { assertion = db.createLocally -> db.user == user;
-        message = "services.bookstack.database.user must be set to ${user} if services.mediawiki.database.createLocally is set true.";
+        message = "services.bookstack.database.user must be set to ${user} if services.bookstack.database.createLocally is set true.";
       }
       { assertion = db.createLocally -> db.passwordFile == null;
         message = "services.bookstack.database.passwordFile cannot be specified if services.bookstack.database.createLocally is set to true.";