docs: update extension guide PyPI upgrade guidance (#3578)

Co-authored-by: root <kinsonnee@gmail.com>

Author: WOLIKIMCHENG

extensions/EXTENSION-DEVELOPMENT-GUIDE.md

@@ -687,7 +687,7 @@ hooks:
 
 **Error**: `Extension requires spec-kit >=0.2.0`
 
-- **Fix**: Update spec-kit with `uv tool install specify-cli --force --from git+https://github.com/github/spec-kit.git`. The bare `specify-cli` package on PyPI is a different, unrelated project — installing it without `--from git+...` will give you a stub CLI that does not include `extension`, `preset`, or other spec-kit commands.
+- **Fix**: Upgrade Spec Kit using the [Upgrade Guide](../docs/upgrade.md). `uv tool install specify-cli --force --from git+https://github.com/github/spec-kit.git` remains available as a source-install fallback. If you installed from PyPI and want to stay on that route, follow the [PyPI upgrade guidance](../docs/install/pypi.md#upgrade).
 
 **Error**: `Command file not found`