fix: Mentions form example not correct display

[shaodahong]

🤔 This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Other (about what?)

🔗 Related issue link

Fixes #19892

💡 Background and solution

📝 Changelog

Language	Changelog
🇺🇸 English
🇨🇳 Chinese

☑️ Self Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

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View rendered components/mentions/demo/form.md

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[codecov]

Codecov Report

Merging #19908 into 4.0-prepare will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           4.0-prepare   #19908   +/-   ##
============================================
  Coverage         97.5%    97.5%           
============================================
  Files              292      292           
  Lines             7247     7247           
  Branches          1963     2002   +39     
============================================
  Hits              7066     7066           
  Misses             181      181

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