Update default nginx config in guide.md (#6471)

update nginx config to avoid wss error when expose code-server using a custom domain and a custom port via nginx.

see also: 
[issue of code-server](https://github.com/coder/code-server/issues/4443)
[different between `$host` and `$http_host`](https://stackoverflow.com/a/76875724)

docs/guide.md

@@ -191,7 +191,7 @@ At this point, you should be able to access code-server via
 
        location / {
          proxy_pass http://localhost:8080/;
-         proxy_set_header Host $host;
+         proxy_set_header Host $http_host;
          proxy_set_header Upgrade $http_upgrade;
          proxy_set_header Connection upgrade;
          proxy_set_header Accept-Encoding gzip;