Changes to custom.densityProperty in examples.md

[Nancy-Salpepi]

For phetsims/qa#1141, the following PhET-iO IDs were made readOnly=true in work done here..

The examples.md doesn't reflect these changes and I'm not sure that @arouinfar was aware that these properties were changed.

Density:

density.introScreen.model.blocks.blockA.materialProperty.customMaterial.densityProperty
density.introScreen.model.blocks.blockB.materialProperty.customMaterial.densityProperty

Buoyancy:

buoyancy.exploreScreen.model.blocks.blockA.materialProperty.customMaterial.densityProperty
buoyancy.exploreScreen.model.blocks.blockB.materialProperty.customMaterial.densityProperty
buoyancy.labScreen.model.block.materialProperty.customMaterial.densityProperty
buoyancy.applicationsScreen.model.objects.block.materialProperty.customMaterial.densityProperty

[arouinfar]

Thanks @Nancy-Salpepi. This will be a fairly major rewrite, as customMaterial.densityProperty is hardly ever editable. I'll wait until after the RC testing is complete to address this issue, in case other issues arise.

[arouinfar]

QA for rc.1 is now complete, so I'll start working on these changes.