Update USING_CUSTOM_EDITORS.md

[geertplaisier]

The rowId argument was missing from the example

[codecov-io]

Codecov Report

Merging #156 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #156   +/-   ##
=======================================
  Coverage   76.83%   76.83%           
=======================================
  Files         112      112           
  Lines        4558     4558           
  Branches      277      277           
=======================================
  Hits         3502     3502           
  Misses        846      846           
  Partials      210      210

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[bencripps]

Awesome, thanks for the PR!