doc/Platforms.md: update for Node.js v12
doc/Platforms.md: update for Node.js v12

kanreisa/Mirakurun,kanreisa/Mirakurun,kanreisa/Mirakurun,Chinachu/Mirakurun,Chinachu/Mirakurun,Chinachu/Mirakurun,Chinachu/Mirakurun