Fix doc preview source directory (#60792)
Summary:
`merge` is the directory with the actual changes, not `master`. Verified by downloading arficats from https://github.com/pytorch/pytorch/pull/60777/checks and searching through the result.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60792
Reviewed By: walterddr
Differential Revision: D29405288
Pulled By: driazati
fbshipit-source-id: 419c943727c00429945c1f116645bfa22fb12456
