Minor update to the nginx config

[freescout-helpdesk]

The following line in the Nginx config

docker-freescout/install/etc/nginx/sites.available/freescout.conf

Line 34 in 73ff9c8

add_header Content-disposition "attachment; filename=$1";

need to be changed to

add_header Content-disposition "attachment; filename=$2";

More info: freescout-help-desk/freescout#3428 (comment)

[freescout-helpdesk]

Already updated.