Fix typo in banner that can appear when making an issue public

What does this MR do and why?

Fixes UI typo.

Closes #350531 (closed)

Screenshots or screen recordings

Before

After

How to set up and validate locally

1. Create a confidential epic.
2. Create a confidential issue linked to the epic created in step 1.
3. Edit the issue and make it public.

Expected result: Banner "This issue cannot be made public because it belongs to a confidential epic."

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.