Remove the obsolete `this` reference in README.md
Remove the obsolete `this` reference in README.md

I was trying to find out what this `var` declaration does, but it seems like it was survived from some copypasta [here](https://github.com/aksonov/react-native-tabs/blob/098b79615031a1825cfe984a1c862327f67dd831/index.js). Not sure if I'm missing something here - but if not, I suppose we could safely remove this?
aksonov/react-native-tabs,aksonov/react-native-tabs