[GHA] Hardcode doc build target to `master` (#66567)
Summary:
According to https://github.com/pytorch/pytorch/blob/f48f20e154f0295cdfd5e1461781f1023287e9eb/.circleci/verbatim-sources/job-specs/job-specs-custom.yml#L46-L48
target should always be master (even on release branches) unless it is a
tagged build
Fixes https://github.com/pytorch/pytorch/issues/66466
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66567
Reviewed By: seemethere
Differential Revision: D31621530
Pulled By: malfet
fbshipit-source-id: d6de2222d0340820555a82ae90b3de22b4dc7b88
