chore(docs): Update nginx example config

This tweak in nginx example should make all
request made by immich web app go through SSL
by default, not requiring other ports than 443
opened to outboud connections.

docs/docs/administration/reverse-proxy.md

@@ -22,6 +22,7 @@ server {
     proxy_set_header X-Real-IP                 $remote_addr;
     proxy_set_header X-Forwarded-For           $proxy_add_x_forwarded_for;
     proxy_set_header X-Forwarded-Proto         $scheme;
+    add_header       'Content-Security-Policy' 'upgrade-insecure-requests';
 
     # enable websockets: http://nginx.org/en/docs/http/websocket.html
     proxy_http_version 1.1;