Improved installation instructions for optional dependencies

[meyer-nils]

While testing the installation for the review (openjournals/joss-reviews#9733) I came across a minor issue regarding the installation instructions:

Installing optional dependencies via

pip install evoxels[torch, jax, dev, notebooks]

may cause issues due to the whitespace or with certain shells like ZSH. This can be improved easily by suggesting

pip install "evoxels[torch,jax,dev,notebooks]"

without spaces and employing quotation marks.

[daubners]

Thank you for the comment. I've updated the readme in #41