docs(module: modal): Update the Form validation example

[ElderJames]

🤔 This is a ...

• New feature
• Bug fix
• Site / documentation update
• Demo update
• Component style update
• Bundle size optimization
• Performance optimization
• Refactoring
• Code style optimization
• Test Case
• Branch merge
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Latest commit:	4a8284c
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 41.35%. Comparing base (d994306) to head (4a8284c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4755      +/-   ##
==========================================
- Coverage   41.39%   41.35%   -0.04%     
==========================================
  Files         600      600              
  Lines       33875    33875              
  Branches     6479     6479              
==========================================
- Hits        14022    14009      -13     
- Misses      18696    18703       +7     
- Partials     1157     1163       +6     

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