Fix line code block in Get started section

[hendzeld]

Fixes code block in point fourth that now appears as one. It should look similar to one in point sixth.

[reviewpad[bot]]

AI-Generated Summary: This pull request includes a minor documentation update. Specifically, it fixes a code block in the "Get started" section of the "contributing.rst" document. The changes make the instructions on creating a virtual environment for installing Python packages more readable by accurately reflecting them as a code block.