Add note about port of X-Forwarded-Host to nginx.conf example

[Chaoses-Ib]

tusd will use X-Forwarded-Host as the host if -behind-proxy is enabled. However, the example Nginx config doesn't put the port number into X-Forwarded-Host, but only the address. This makes the URL returned by tusd to be on port 80/443 instead of what Nginx is actually listening on.

Changing proxy_set_header X-Forwarded-Host $host to proxy_set_header X-Forwarded-Host $host:$server_port can fix this problem. But this leads to the occurrence of :80/:443 in the returned URL even the client doesn't put :80/:443 in the Host header, which can be a little weird. So I just add a comment about this.

Related issue: #136

[Acconut]

Thanks for the PR! We can also consider adding a logic in tusd's URL generation to drop the default ports for HTTP(S) if they are used.