it says:
/media/gantry5/engines/nucleus
to
TEMPLATE_DIR/custom/engine
the path in that other link
ROOT/media/gantry5/engines/nucleus/particles/ and paste the copies to TEMPLATE_DIR/custom/particles
is correct for particles. The file you want to override is not a particle ...
This maybe also of interest for you:
gantry.org/blog/gantry-5-developers-perspective