chore: typo fix on singleton.js

[zhennann]

clent -> client

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8f54fe9) 99.86% compared to head (9bc8323) 99.86%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5279   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files          36       36           
  Lines        3608     3608           
  Branches      517      517           
=======================================
  Hits         3603     3603           
  Misses          5        5           

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[fengmk2]

@zhennann Thanks!