Clean up getInputFromUser example
Current Example:
```
UI.getInputFromUser("What's your favorite design tool?", {
  type: UI.INPUT_TYPE.selection
  possibleValues: ['Sketch']
}, (err, value) => {
  if (err) {
    // most likely the user canceled the input
    return
  }
})
```
Corrected Example:
```
UI.getInputFromUser("What's your favorite design tool?", {
  type: UI.INPUT_TYPE.selection,
  possibleValues: ['Sketch', 'Paper']
}, (err, value) => {
  if (err) {
    // most likely the user canceled the input
    return
  }
})
```

The corrected example adds a missing comma and demonstrates that there can be other possible values other than Sketch 😂.

One other small thing while I am commenting on this..
<img width="532" alt="screen shot 2018-12-19 at 11 30 54 pm" src="https://user-images.githubusercontent.com/4199296/50270378-2f27a700-03e6-11e9-9472-59a8010bd74b.png">
It feels like the input could be just a bit taller? Also, I think a NSPopUpButton that pulls down would look nicer but thats just my 2 cents.