hierarchy-separator: fix node highlight

[yannbf]

Issue: N/A

What Changed

The highlighting of the rule was being applied to the entire meta. Now it does only for the title node:

Checklist

Check the ones applicable to your change:

• [ ] Ran yarn update-all
• [x] Tests are updated
• [ ] Documentation is updated

Change Type

Indicate the type of change your pull request is:

• [ ] maintenance
• [ ] documentation
• [x] patch
• [ ] minor
• [ ] major

[github-actions[bot]]

:rocket: PR was released in v0.3.0 :rocket: