Update filename to workers.py file in documentation

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

Reviewed By: gkistanova

Differential Revision: https://reviews.llvm.org/D94886