Fix documentation on how CI_COMMIT_REF_PROTECTED works

What does this MR do?

The behavior of CI_COMMIT_REF_PROTECTED is the following:

• True if CI_COMMIT_BRANCH and branch is protected,
• True if CI_COMMIT_TAG and tag is protected,
• False otherwise.

This corrects the documentation by using the word "reference" and not branch.

Conformity

• Changelog entry (not required, doc only)
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team