Docs: Improve capitalization and grammar in Omnibus GitLab README

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This MR updates the Omnibus GitLab README to improve clarity and consistency by:

Correcting capitalization in section titles (e.g., “After Installation” → “After installation”).

Refining grammar in troubleshooting and security contributor sections.

Ensuring consistent formatting across headings and descriptions.

Submitted as part of the GitLab Hackathon August 2025.