[Go SDK]: Remove anonymous function from fileio.ReadMatches example

[johannaojeling]

Fixes #29185 and removes the anonymous function used in the fileio.ReadMatches doc example.

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[codecov]

Codecov Report

Merging #29186 (0e02e6a) into master (c2816c8) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #29186      +/-   ##
==========================================
+ Coverage   38.36%   38.38%   +0.02%     
==========================================
  Files         687      687              
  Lines      101745   101745              
==========================================
+ Hits        39033    39054      +21     
+ Misses      61135    61112      -23     
- Partials     1577     1579       +2     

Flag	Coverage Δ
go	53.47% <ø> (+0.05%)	⬆️

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see 3 files with indirect coverage changes

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