fix(module:drawer): drawer content cannot scroll

[hsuanxyz]

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
[x] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #2119

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #2120 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2120      +/-   ##
==========================================
- Coverage      96%   95.99%   -0.01%     
==========================================
  Files         477      477              
  Lines       11618    11618              
  Branches     1550     1550              
==========================================
- Hits        11154    11153       -1     
  Misses        132      132              
- Partials      332      333       +1

Impacted Files	Coverage Δ
components/tabs/nz-tabset.component.ts	95.72% <0%> (-0.86%)	⬇️

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