doc/user/permissions: update option and menu names

What does this MR do?

This fixes doc/user/permissions.md documentation.

Are there points in the code the reviewer needs to double check?

The reviewer can check that the name of the option is now Public pipelines and that this option is in Project Settings > CI/CD Pipelines.

Why was this MR needed?

Because the previous name of the option and of the menu had not been updated in doc/user/permissions.md when they were changed.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

This is a documentation only MR, so I don't think it needs to pass all these acceptance criteria.

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

I don't know of any issue number.