fix(module:checkbox): fix checkbox a11y error

[vthinkxie]

close #3000

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?

[x] 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
[x] No

Other information

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[codecov]

Codecov Report

Merging #3009 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3009      +/-   ##
==========================================
+ Coverage   97.35%   97.35%   +<.01%     
==========================================
  Files         550      550              
  Lines       11452    11454       +2     
  Branches      813      813              
==========================================
+ Hits        11149    11151       +2     
  Misses        190      190              
  Partials      113      113

Impacted Files	Coverage Δ
components/checkbox/nz-checkbox.component.ts	100% <100%> (ø)	⬆️

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