fix(module:tree-select): update nzPlacement demo

[JackYumg]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] 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
[x] 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

[zorro-bot[bot]]

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[codecov[bot]]

Codecov Report

Merging #7551 (341f540) into master (64b7389) will decrease coverage by 0.03%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #7551      +/-   ##
==========================================
- Coverage   91.75%   91.72%   -0.04%     
==========================================
  Files         502      502              
  Lines       16699    16706       +7     
  Branches     2604     2607       +3     
==========================================
+ Hits        15322    15323       +1     
- Misses       1047     1050       +3     
- Partials      330      333       +3     

Impacted Files	Coverage Δ
components/tree-select/tree-select.component.ts	90.49% <66.66%> (-0.92%)	:arrow_down:
components/tabs/tab-nav-bar.component.ts	86.52% <0.00%> (-1.42%)	:arrow_down:
components/core/util/text-measure.ts	92.98% <0.00%> (+0.87%)	:arrow_up:

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