I think there is a typo in the tutorial. At the end of the first line is a colon too much. Don’t forget to rebuild!
For https installations It should be:
proxy_pass http://unix:/var/discourse/shared/standalone/nginx.http.sock;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header Host $host;
proxy_http_version 1.1;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto "https";
Remove the last line if you do not use https. If this does not work, please type the following at the console and show us the result:
ls -al /var/discourse/shared/standalone/nginx.http.sock
updatedb && locate nginx.http.sock
If “locate” ist not found, you can install it:
apt-get install locate