heroes3.yaml: replace Windows demo with MacOS demo

The MacOS demo contains the all the non-graphical assets from the
complete edition. The only graphics missing are those for towns not
included in the demo. There exists a vcmi mod that disables the missing
assets and thus makes the game playable just with the data from the
MacOS demo.

The Windows Demo on the other hand only contains the base RoE game without
expansion and is incompatible with vcmi because texts have different IDs and
many assets are plainly missing. vcmi never worked with this demo.