Update issue template for Sketch -> Snack rename

Summary:
[We changed the name from Sketch to Snack](https://blog.expo.io/expo-sketch-expo-snack-a444f8dec72b)

It's just some text in the issue template :)

- Make it clear on the Snack website which version of react-native you are using. Right now we make it possible to pick SDK versions but it's not at all obvious which react-native version that maps to.
Closes https://github.com/facebook/react-native/pull/13304

Differential Revision: D4833399

Pulled By: ericvicenti

fbshipit-source-id: 25b1fe8b4b1ac7db6737e1ba8f611ad4d8dc3804