docs(py): update Python README.md for package installation

[songfeil]

Currently when installing with the subdirectory parameter, pip will return the following error:

ERROR: querychat@ git+https://github.com/posit-dev/querychat#subdirectory=pkg-py from git+https://github.com/posit-dev/querychat#subdirectory=pkg-py does not appear to be a Python project: neither 'setup.py' nor 'pyproject.toml' found.

Looks like this is due to missing 'pyproject.toml' file, which is currently placed under the root directory. This change redirect the git repo to not use subdirectory, so that pip can discover and install the package correctly.

[chendaniely]

Thanks @songfeil for the update. We're in the process up creating and updating the package website page, but this was an oversight as things are moving around in #24 as we move into a monorepo structure (#17).

For reference the package website information is moved and being captured in PR#30