Stem directory authority documentation is outdated

On https://stem.torproject.org/api/descriptor/remote.html the stem.descriptor.remote.DirectoryAuthority and get_authorities() documentation is outdated:

• the hardcoded into tor link points to the wrong line: https://gitweb.torproject.org/tor.git/tree/src/or/config.c#n819
• the date for the authority list is in 2014, which seems very old