fix(doc): remove incorrect --with flag in install command

[hassec]

The installation instructions incorrectly pass a --with dev to the pip install cfspopcon.

While poetry has a --with flag, pip doesn't have a --with flag. And poetry dev dependencies aren't something one can install via a pip command, so just removing this.

I've tested that just a simple

pip install cfspopcon
radas -d ./radas_dir

still works.