fix typo in contributing.rst

[atrisovic]

• Fix small typos in contributing.rst
• Add tox in docs requirements which is needed for people who contribute to the docs to test it on their machine.

[skasberger]

The contributing.rst file changes are fine.

The requirements changes need some changes, as it is not allowed to create a PR against the master branch (only doc changes) for such changes. I suggest another way to tackle it: add tox to the development.txt and remove it from tests.txt. then tox is available for everyone who does some development work, not matter if only docs, or no docs or with linting and so on.

[atrisovic]

Ok, good point, I fixed it now. It will still be one commit. I will create a new PR for tox as you suggested.