nixos/zoneminder: Fix nginx config check

NixOS wouldn't build because the nginx config checker fails.

Location without a trailing slash "could allow an attacker to read file
stored outside the target folder.", source:
https://github.com/yandex/gixy/blob/master/docs/en/plugins/aliastraversal.md

Shouldn't change the behaviour according to
https://serverfault.com/questions/607615/using-trailing-slashes-in-nginx-configuration/607731#607731