examples: fix "main" entry point in package.json

[DLehenbauer]

The "main" entry point for the samples previously pointed to the non-existent 'index.js'.

Fixing this allows people to run samples with the shorthand node ., which consults ./package.json for the entry point.

Of course, node ./dist/main.js still works too.

I did not update the readme or examples.md to use node ., since I thought this was less obvious, but can if desired.

[DanielRosenwasser]

Great idea - thank you!