docs: drop the upstream remote from CONTRIBUTING

profile_sync.py finds and fetches the project's main by itself. The
documented --base upstream/main failed in any clone without that remote,
where the same command without --base passes.

scripts/generate_readme.py

@@ -981,7 +981,7 @@ the checks CI runs on a pull request (Python 3.12 or newer):
 pip install pyyaml "jsonschema[format-nongpl]" markdown pyte hypothesis vermin
 python scripts/validate_pr.py --changed          # the BIOS files the branch adds
 python scripts/validate_schemas.py --source-only # profiles and platform files
-python scripts/profile_sync.py --added --base upstream/main --strict  # each profile the branch adds
+python scripts/profile_sync.py --added --strict  # each profile the branch adds
 python scripts/refresh_data_dirs.py  # data directories the pack-building tests read
 python scripts/generate_site.py       # the pages the site tests read
 npm ci --prefix website && npm run build --prefix website  # the built site, Node 24
@@ -990,10 +990,9 @@ python scripts/validate_site.py --site-dir website/dist  # links and page budget
 vermin --no-tips -t=3.8- --violations install.py         # install.py stays on Python 3.8
 ```
 
-`upstream` is this repository as a remote of your fork
-(`git remote add upstream https://github.com/Abdess/retrobios`, then
-`git fetch upstream`). A profile you only edit is not held to `--strict`: its
-older refs are reviewed by hand.
+`profile_sync.py` compares the branch with this repository's `main`, and
+fetches it in a fork's clone. A profile you only edit is not held to
+`--strict`: its older refs are reviewed by hand.
 
 The full pipeline is not needed for a contribution: it rebuilds every pack
 in `dist/`, which is why maintainers run it before a release.