Add installation command for knowledge-clustering

[samvang]

When I was trying to install knowledge-clustering on Ubuntu 24.04.3 LTS, I needed to also run pipx install knowledge-clustering, not just pipx install knowledge. When I ran pipx install knowledge, I got the following message:

No apps associated with package knowledge or its dependencies. If you are
attempting to install a library, pipx should not be used. Consider using pip
or a similar tool instead.

I propose to add that command to the readme file.

[remimorvan]

You're right, it was a typo: actually the PyPI package knowledge is not related to the LaTeX package knowledge or to knowledge-clustering, you can run pipx uninstall knowledge to remove it. Sorry for that!