Fix smokescreen example url in smokescreen.html.erb

[irreal]

Summary of changes

Update incorrect url to example repo for using Smokescreen

Related GitHub and Fly.io community links

n/a

Notes

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[andie787]

thanks!

[irreal]

Hi @andie787 , I'm not sure who maintains the actual Smokescreen example that I fixed the url to.
It was out of date and not working according to the Http Proxy spec. So I fixed it in two separate PRs: fly-apps/smokescreen#2 and fly-apps/smokescreen#3

It might be useful to get those merged in, and then you could even remove the outdated tag from this document, as I now brought the example up to date and verified everything is indeed working great. I'm using it in production right now.