Hey,
I have reported the issue in the docs, which we will correct soon!
In the mean time, you can find the checkerboard within the MetaHuman plugin folder of your engine install, eg the default location is:
C:\Program Files\Epic Games\UE_5.2\Engine\Plugins\Marketplace\MetaHuman\Content\StereoCaptureTools\stereo-calib-grid-16v11h-7p5mm.png
Hope that helps,
James