readme: add parens where needed

[wmertens]

I got a little confused why my copy/paste wasn't working

[a-kenji]

I don't believe they are strictly needed, are they?
They can be needed depending on the context, but not having them is more readable and understandable IMO.

[wmertens]

I needed them because I was the listing the packages needed in my flake, but feel free to close this if you don't think it adds value.