doc: Mention show(example) at the very beginning of the tutorial

[blinry]

Hey :) We tried following the Selectors tutorial. When we tried running the code after step 2, we were surprised that we didn't get any output.

For very new users like ourselves, it would be good to mention show(example) at the very beginning, before step 6, so you can run the steps individually, and see your progress.

[gumyr]

Done. Thanks for pointing this out.