docs: remove RELEASE_VERSION from trivy.repo

[Blarc]

Description

When adding repo configuration to /etc/yum.repos.d/trivy.repo, RELEASE_VERSION should not be added to repo baseurl.

This is also documented in: https://aquasecurity.github.io/trivy-repo/.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[CLAassistant]

All committers have signed the CLA.

[knqyf263]

Thanks