docs: clean up demo line number highlights

[andretchen0]

Problem

A few demos had line numbers written like this:

{2,4,5,6,7,8,9,10,11,20}

Solution

Write them like this ...

{2,4-11,20}

... for easier readability/maintenance.

Problem

The demo for materials/custom-shader-material had changed, but the line number highlights weren't updated.

Solution

Update the line number highlights.

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