Update name to workers.py file in documentation

[dyung]

Summary

Commit be9f322e8dc530a56f03356aad31fa9031b27e26 moved the list of workers from slaves.py to workers.py, but the documetation in "How To Add A Builder" was never updated and now references a non-existing file. This fixes that.

[gkistanova]

Thanks, Douglas, for fixing this!

llvm/docs/HowToAddABuilder.rst

#. Make sure you can actually start the buildbot-worker successfully. Then set
    up your buildbot-worker to start automatically at the start up time.  See the
    buildbot documentation for help.  You may want to restart your computer
    to see if it works.
 
 #. Send a patch which adds your build worker and your builder to
    `zorg <https://github.com/llvm/llvm-zorg>`_. Use the typical LLVM 
    `workflow <https://llvm.org/docs/Contributing.html#how-to-submit-a-patch>`_.
 
-   * workers are added to ``buildbot/osuosl/master/config/slaves.py``
+   * workers are added to ``buildbot/osuosl/master/config/workers.py``
    * builders are added to ``buildbot/osuosl/master/config/builders.py``
 
    Please make sure your builder name and its builddir are unique through the
    file.
 
    It is possible to allow email addresses to unconditionally receive
    notifications on build failure; for this you'll need to add an
    ``InformativeMailNotifier`` to ``buildbot/osuosl/master/config/status.py``.