fix(module:slider): fix the style of step-dots in vertical mode

[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?

[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

[zorro-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 91.59%. Comparing base (2000294) to head (7e8f747).
Report is 1 commits behind head on master.

Files	Patch %	Lines
components/slider/step.component.ts	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8494      +/-   ##
==========================================
- Coverage   91.59%   91.59%   -0.01%     
==========================================
  Files         533      533              
  Lines       18354    18356       +2     
  Branches     2799     2800       +1     
==========================================
+ Hits        16812    16813       +1     
  Misses       1226     1226              
- Partials      316      317       +1     

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[Nicoss54]

LGTM

[Laffery]

LGTM