Fix: Text in other languages was removed from anchors

[AmosHuKe]

Description of what this PR is changing or adding, and why:

Markdown

## Hello! 你好！ हैलो مرحبا -

If the header contains text in other languages, those languages are removed:

<div class="header-wrapper">
  <h2 id="hello">Hello! 你好！ हैलो مرحبا -</h2>
  <a class="heading-link" href="#hello" aria-label="Link to 'Hello! 你好！ हैलो مرحبا -' section">#</a>
</div>

The expected HTML looks like this:

<div class="header-wrapper">
  <h2 id="hello-你好-हल-مرحبا">Hello! 你好！ हैलो مرحبا -</h2>
  <a class="heading-link" href="#hello-你好-हल-مرحبا" aria-label="Link to 'Hello! 你好！ हैलो مرحبا -' section">#</a>
</div>

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[AmosHuKe]

CC @parlough 😄

[sfshaza2]

/gcbrun

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 1fee75b):

https://flutter-docs-prod--pr11151-patch-anchors-1-pac6eiaj.web.app

[parlough]

I didn't consider this, thanks!