docs: activate the virtualenv created by uv venv

[dajiaohuang]

Description

The Local Setup instructions run uv venv, which creates .venv, then try to activate ./pgcli-dev/bin/activate. Following these commands in a clean directory fails with No such file or directory and leaves the environment inactive.

Activate .venv/bin/activate instead, matching the preceding command. No runtime behavior changes.

Validation (Linux, Python 3.12):

• Reproduced the original activation failure (exit 1); the corrected command succeeds and sys.prefix matches VIRTUAL_ENV.
• Rendered CONTRIBUTING.rst and changelog.rst with docutils --halt=warning.
• ruff check, ruff format --check, and git diff --check pass.

Checklist

• I've added this contribution to the changelog.rst.
• I've added my name to the AUTHORS file (or it's already there).
• I installed pre-commit hooks (pip install pre-commit && pre-commit install).
• I verified that my changes work as expected.
• Please squash merge this pull request.

[j-bennet]

Thanks @dajiaohuang, good catch.