[doc] Apply buildbot worker terminology change: slave->worker

Recently [1], there was an upgrade to the version of buildbot being
deployed. The new setup will still work with old buildslaves but I
thought it might be a good idea to update the documentation to reflect,
that you now can use a newer buildbot version to when setting up your
worker (formely known as slave).

The upgrade from buildbot 0.8.5 to 2.8.5 went a long with a transition
to a new "worker" terminology [2] which is also reflected by this
change.

[1]: http://lists.llvm.org/pipermail/llvm-dev/2020-October/145629.html
[2]: http://docs.buildbot.net/0.9.12/manual/worker-transition.html

Reviewed By: gkistanova

Differential Revision: https://reviews.llvm.org/D89230