Fixed editable install command to work universally with most shells

[akashpalrecha]

This is the same fix as in fastai/fastai2#80

The existing command in the README for an editable install is: pip install -e .[dev].
This does not work in the zsh shell. And since MacOS is now defaulting to zsh, it becomes important for it to work with it as well.

Changing it to: pip install -e ".[dev]" makes it work with both bash and zsh (on ubuntu, macOS and Windows).

This GitHub issue adds more light into the issue, and it's potential solutions: mu-editor/mu#852.
This comment concludes that adding double quotes is a universal solution for most platforms and shells: mu-editor/mu#852 (comment)

[sgugger]

Thanks!