Fix missing http2

Seems OliveTin require HTTP2 to be enabled, this will fix nginx configuration.

docs/modules/ROOT/examples/reverse-proxies/etc/reverse_proxy_nginx_dns.conf

@@ -1,5 +1,6 @@
 server {
     listen 443 ssl;
+    http2 on;
 
     ssl_certificate "/etc/nginx/conf.d/server.crt";
     ssl_certificate_key "/etc/nginx/conf.d/server.key";