Typo on Platform.md page

[Robert Chalmers]

npm install -g cordova
cd ./build/mobile/
npm install
cordova add android

should read
npm install -g cordova
cd ./build/mobile/
npm install
cordova platform add android

[John Sim]

Thanks - next update I'm pushing out an overhaul to the readme to match correct npm script names etc..

[sunk818]

@ShanghaiTimes It is simple to edit the readme.md and submit a pull request like I just did.