[docs] Improve guidance on installing the correct buildbot-worker version

The docs currently state that buildbot-worker 2.8.5 is used. However,
looking at the buildbot release notes
<http://docs.buildbot.net/current/relnotes/index.html>, this release
doesn't seem to exist (it's not on PyPI either). The document also links
to http://trac.buildbot.net/ as a source to retrieve an old version and
install it manually, but this is dead.

Instead, drop that advice and suggest pip3 install
buildbot-worker==2.8.4, as done by the Dockerfiles in
llvm-zorg/buildbot/google/docker/*.

Differential Revision: https://reviews.llvm.org/D144464