docs: fix responsive

[wzhudev]

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?

[ ] 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?

Fix a responsive design bug by making the menu collapse at a wider screen width.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[netlify]

Deploy preview for ng-zorro-master ready!

Built with commit ce92728

https://deploy-preview-3624--ng-zorro-master.netlify.com

[codecov]

Codecov Report

Merging #3624 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3624   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files         685      685           
  Lines       14044    14044           
  Branches     1866     1866           
=======================================
  Hits        13406    13406           
  Misses        231      231           
  Partials      407      407

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[vthinkxie]

LGTM