Maybe no // Output: was provided because you wouldn't want this to fail in a source tree that was already released. It could lead people to believe something about the tls package had broken when in fact was just a website or a website's certificate that had changed. Perhaps a comment about how this might fail in the future if someone like me stumbles on it wanting to run it locally as a main program.
I ran into it because I was getting an odd TLS certificate failure on my darwin system and I had seen work recently regarding others having TLS issues too with their darwin systems. When I ran this locally and it failed, I was misled in thinking it was something about my darwin release or the golang stdlib. Lucky I tried the same program on a Linux machine before filing a bug and found it fail there too. That's when I found there was a system certificate that would make it work, just not the one hard coded in the example.