docs: correct typo in `index.md` (#8150)
This was added in [#5426](https://github.com/astral-sh/uv/pull/5426)
The output that a python shell was started, but the command will only
print out the version because of the `--version` flag.
I think the section `Download Python versions as needed` should be
reverted back to `Download Python versions on demand` or something
similar because that's what the command will do. It will download the
given version if not already installed.
