tour: Improve concurrency example

[xlr-8]

Context: https://tour.golang.org/concurrency/5

It took me a bit of time to understand what was going on on this step.

Until I realized that the go func() was actually waiting for the Fibonacci sequence function to send information to the channel, and was also stopping it by sending something into quit.

I simply added some print to figure this out, but I guess it might be worth improving the description or code snippet.