docs: migration description

[samsonkumawong]

Update migration-v15.en-US.md from 13.x -> 14.x to 14.x -> 15.x

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?

Incorrect migration description

Issue Number: N/A

What is the new behavior?

Updated correct migration description

Does this PR introduce a breaking change?

[ ] Yes
[ *] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #7890 (97034c0) into master (1c48745) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 97034c0 differs from pull request most recent head 8805b2c. Consider uploading reports for the commit 8805b2c to get more accurate results

@@            Coverage Diff             @@
##           master    #7890      +/-   ##
==========================================
+ Coverage   91.81%   91.82%   +0.01%     
==========================================
  Files         506      506              
  Lines       17302    17302              
  Branches     2747     2747              
==========================================
+ Hits        15886    15888       +2     
+ Misses       1130     1129       -1     
+ Partials      286      285       -1     

see 1 file with indirect coverage changes

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[simplejason]

LGTM