Fix sequence value for merge key

[goccy]

resolve #340

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 64.13043% with 33 lines in your changes missing coverage. Please review.

Project coverage is 77.15%. Comparing base (aeed806) to head (3d450da).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #603      +/-   ##
==========================================
- Coverage   77.19%   77.15%   -0.05%     
==========================================
  Files          21       22       +1     
  Lines        7850     7879      +29     
==========================================
+ Hits         6060     6079      +19     
- Misses       1369     1381      +12     
+ Partials      421      419       -2     

[morningvera]

I was actually migrating a repo from gopkg.in/yaml.v3 to this library yesterday and was trying to use the merge operator like this. Thanks for releasing it today @goccy !