old experimental version 0.3.3.x https://www.torproject.org/docs/debian.html.en
You've forgotten to turn 0.3.3.x into 0.3.4.x under experimental version for stretch.
While you there how about putting in plain brief english what to do if you get an error because apt-transport-https isn't installed?
You could add 'apt install apt-transport-https' in the style of the other commands.
One reason to muck around with tor was to learn Linux but repeatedly there are preumptions of prior knowledge which cause me grief and others too which is discouraging. Have someone review things who isn't into secret lingo. Rob