Change: Corrected one product name in README

[cfi-gb]

What:

Change Trial to TRIAL as initially used in #50 but changed afterwards.

Why:

Follow the naming conventions like seen on e.g. https://www.greenbone.net/en/testnow/ or in Confluence

How:

N/A

Checklist:

• [ ] Tests
• [ ] CHANGELOG.md entry
• [ ] Documentation