onBrokenAnchors fails with unicode

[Zamiell]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

I tried out the new onBrokenAnchors feature, but ran into a bug: it gives a false positive with the following header:

### Schrödinger's Cat Principle

The link:

- First, see the section on [Schrödinger's Cat Principle](foo.mdx#schrödingers-cat-principle).

Your environment

• Public source code: https://github.com/hanabi/hanabi.github.io/
• Public site URL: hanabi.github.io
• Docusaurus version used: 3.1.0

Self-service

• I'd be willing to fix this bug myself.

[slorber]

Thanks for reporting, will be fixed in #9732