docs: fix minor code block issue for local installation in CONTRIBUTING.rst

[dluks]

Fixes a typo which caused code block rendering to fail. Also adds quotes around .[all] as pip install -e .[all] fails in some environments. E.g.:

$ python -m pip install -e .[all]
zsh: no matches found: .[all]

$ python -m pip install -e ".[all]"
... # (works)

• [x] Fully documented, including docs/history.rst for all changes and docs/rioxarray.rst for new API

:point_up: I didn't add anything to docs/history.rst as it doesn't appear to be the convention when updating documentation.

[snowman2]

Thanks @dluks :+1: