Fixing Shell highlight in README.md

[filipstachura]

This is just small fix in documentation.

General policy:

• If applicable - add instructions for testing
• If there’s no issue, create it. Each issue needs to be well defined and described.
• All interaction with a user, user-facing messages, plots, reports etc. are written from the perspective of the person using or receiving it. They are understandable and helpful to this person. If a user sees an error message, there is a call to action, i.e. the user knows what to do to fix it.
• README, other documentation and code comments that we have is updated with all information related to the change.
• All code has been peer-reviewed before merging into any main branch
• All changes have been merged into the main branch we use for development.
• Continuous integration checks (linter, unit tests, integration tests) are configured and pass.
• Optional: unit tests added for all new or changed logic.
• All task requirements satisfied. If not describe it here. The reviewer is responsible to verify each aspect of the task.
• Change covers only things in task. Please create new PR if you want to fix something else.