docs: use quotes in pip install instruction

[thatlittleboy]

A small PR to encourage the use of quotes around the package name during pip install. (README.md)

Under zsh,

$ pip3 install -U polars[pyarrow]

will throw an error

zsh: no matches found: polars[pyarrow]

which might throw some beginners off.

The more robust solution is to put quotes around the package name so globbing doesn't happen. And this is compatible with bash as well.

[ritchie46]

Thanks!