docs: update venv activation for windows (#1836)
## Summary
Follow on to PR https://github.com/astral-sh/uv/pull/1811 as part of
issue https://github.com/astral-sh/uv/issues/1750. This change updates
the windows venv activation to work for both Powershell and Command
Prompt instead of just Powershell. This also aligns with what `uv venv`
displays.
