docs(module:form): fix form-register demo

[HyperLife1119]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Application (the showcase website) / infrastructure changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.45%. Comparing base (830b4b3) to head (ef425ad).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9459   +/-   ##
=======================================
  Coverage   91.44%   91.45%           
=======================================
  Files         563      563           
  Lines       19813    19827   +14     
  Branches     3175     3176    +1     
=======================================
+ Hits        18118    18132   +14     
  Misses       1350     1350           
  Partials      345      345           

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[Laffery]

LGTM