Fix learn more link in Onion Auth prompt

The Learn more link in the "requesting your private key" dialogue leads to a 404 page:

https://2019.www.torproject.org/docs/tor-manual-dev.html.en-US#client…

It should go to

https://tb-manual.torproject.org/onion-services/ or https://support.torproject.org/onionservices/client-auth/

via https://blog.torproject.org/comment/288070#comment-288070