docs(README): open recommended installation details by default

[glmlm]

This PR makes the recommended installation method expand by default by adding open to its <details> tag.

Expanding only the recommended option makes it a bit easier to read.

[brianhuster]

It was originally recommended as it was also the one I used, but as Nvim is going to have a built-in plugin manager, I don't think we need to recommend it anymore

[glmlm]

That makes sense, thanks for the explanation.

My intention with this PR wasn’t really to push a specific recommendation.
When I first read the installation section, I had a bit of trouble spotting a concrete example.
It felt clearer to me where to start when one option was expanded by default.

That said, I totally understand not wanting to recommend a particular plugin manager.
I’m happy to adjust this if you think keeping everything collapsed (or expanding a more neutral example) fits the project better.

[brianhuster]

When 0.12 is released, I will "recommend" vim.pack. Thank you but right now, I don't want any changes to this