Fix tiny shell typo in tests.rst

[jeffjennings]

Single typo correction, just in case this catches people reading through the Guide.

From the pip docs,

pip install .

works but not

pip install .[x]

Instead it should be

pip install ".[x]"

[Cadair]

If you need to quote this is shell-specific (you don't on bash you do on zsh etc) but I am happy to merge it as it also works on bash 😅

[Cadair]

Thanks!!

[jeffjennings]

Thank you!