examples/nginx_proxy: recommend `proxy_buffering off'

examples/nginx_proxy: recommend `proxy_buffering off'

public-inbox-httpd has always been designed to handle slow
clients efficiently via non-blocking sockets and epoll|kqueue.

Thus the proxy buffering capabilities of nginx were a needless
waste of memory and filesystem traffic and increases response
latency.

nginx does provide an HTTPS-capable reverse-proxy to talk to
varnish, however, any other HTTPS-capable reverse proxy works,
too.