d3 v5 - update to d3.json (Chapter 14)

Hey! Thanks for the great work with the book - making my way through it now. Similar to Issue #34 , a change needs to be made when making a json request. 

On line 29, instead of 
`d3.json("us-states.json", function(json) {`

Should instead now be
```
d3.json("us-states.json")
  .then(function(json){	
```

Cheers!