Correct description of installation requirements in contributing.md

[SabineHaas]

Fix #Issue

Changes proposed in this pull request:

• Corrected description of installation requirements in contributing.md

The following steps were realized, as well (required):

• Update the CHANGELOG.md
• Check if full simulation tests pass locally (EXECUTE_TESTS_ON=master pytest)

Also the following steps were realized (if applies):

• Use in-line comments to explain your code
• Write docstrings to your code
• For new functionalities: Explain in readthedocs
• Write test(s) for your new patch of code

Please mark above checkboxes as following:

• Open
• Done

In case of an error due to linting, run black . --exclude docs/ and push your changes.
Note that in case you do not fix a whole issue you should start your PR with Address #xyz.

_{For more information on how to contribute check the CONTRIBUTING.md.}