fix(module:back-top): fix style

[cipchk]

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

[codecov]

Codecov Report

Merging #1125 into 0.7.0 will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            0.7.0    #1125      +/-   ##
==========================================
+ Coverage   97.99%   98.01%   +0.01%     
==========================================
  Files         156      156              
  Lines        5448     5448              
  Branches      656      656              
==========================================
+ Hits         5339     5340       +1     
+ Partials      109      108       -1

Impacted Files	Coverage Δ
components/back-top/nz-back-top.component.ts	100% <ø> (ø)	⬆️
components/table/nz-table.component.ts	95.23% <0%> (+0.68%)	⬆️

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