gw-nic: Correct syntax highlight for nginx

docs/services/gateway-nic.md

@@ -309,7 +309,7 @@ PartOf=netfilter-persistent.service
 
 To mitigate the issue of the complaints from ISPs and the regulation authorities caused by the gateways in USTCnet responding to the requests for `ustclug.org`, which is a unregistered domain in China MIIT, we make nginx listen on an alternative port 81/444 for HTTP and HTTPS respectively, to respond to requests for `lug.ustc.edu.cn` only, and rejecting the handshake for any other domain.
 
-``` title="/etc/nginx/sites-available/default"
+```nginx title="/etc/nginx/sites-available/default"
 server {
   listen 81 default_server;
   listen [::]:81 default_server;