Avoid confusion from "nginxproxy.fly.dev" not being a real address

[benhoyt]

Currently this doc is confusing as it sounds like nginxproxy.fly.dev is a real domain name that your CNAME should point to ... but it's really talking about <yourapp>.fly.dev. I was confused by this and set up the wrong CNAME at first, and it looks like I'm not the only one.

[jackie-pc]

I hit this exactly. Of course if I used my brain it would have been obvious. But in a rush, I hit this issue!

[andie787]

fixed by #1699