zoneminder: fix nginx config

For some reason it doesn't seem to load things in the cache directory
properly without this slash.

Looks like this regression may have been introduced in:

  commit 19851ec1


  nixos/zoneminder: Fix nginx config check

Cc: Daniel Schaefer <git@danielschaefer.me>
Cc: Peter Hoeg <peter@hoeg.com>

Signed-off-by: William Casarin <jb55@jb55.com>