Update links in Stem tutorials

I noticed that a few places could be updated in the Stem tutorials since:

• the documentation for running Tor relays has moved to a new URL and the old site is no longer updated, and
• Trac ticket 9850 has been closed.

I made these updates here: https://github.com/torproject/stem/pull/27

Please take a look. Thank you!

Trac:
Username: hannelores