electron.rst#bundling-npm-packages is outdated

[Simon-Laux]

https://github.com/flatpak/flatpak-docs/blob/master/docs/electron.rst#bundling-npm-packages

the usage of the node/npm tool changed, see it's readme for details: https://github.com/flatpak/flatpak-builder-tools/tree/master/node

this is probably a "good first issue"