doc: fix `mapStream` function error

[zdyxry]

[codecov-commenter]

Codecov Report

Merging #64 (975f0c2) into main (f7fb299) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #64   +/-   ##
=======================================
  Coverage   99.45%   99.45%           
=======================================
  Files          10       10           
  Lines         364      364           
=======================================
  Hits          362      362           
  Misses          2        2           

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