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 19851ec1fce3fabec3c6f951c3fb7d5814d24691
  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>

nixos/modules/services/misc/zoneminder.nix

@@ -263,7 +263,7 @@ in {
                 }
 
                 location /cache/ {
-                  alias /var/cache/${dirName};
+                  alias /var/cache/${dirName}/;
                 }
 
                 location ~ \.php$ {