fix comment in mode.go

[plsmaop]

EnableJsonDisallowUnknownFields => EnableJsonDecoderDisallowUnknownFields

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[codecov]

Codecov Report

Merging #2129 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2129   +/-   ##
=======================================
  Coverage   98.74%   98.74%           
=======================================
  Files          40       40           
  Lines        2236     2236           
=======================================
  Hits         2208     2208           
  Misses         14       14           
  Partials       14       14

Impacted Files	Coverage Δ
mode.go	100% <ø> (ø)	⬆️

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