Using Nginx how do I redirect single URIs from http to https?

HTTPS and normal HTTP will come in through different ports, so really what you want to do is just redirect any normal HTTP requests that you want to be served over HTTPS to HTTPS. So the following:

server {
  listen                      80;
  server_name                 _;

  location / {
    rewrite ^/(pro)$ https://$host/$1;
  }
}

... should not cause a redirect loop, because it is forwarding only on :80, and it is forwarding TO :443 (which is what HTTPS requests will come in as, by default).