Add missing prop-types import (#8)

Author: jrnt30

text/6.md

@@ -428,6 +428,7 @@ that parses our [query document](2.md#document) string, converting it into a str
 that we can pass to Apollo—now we can give it to Apollo’s `<Query>` component:
 
 ```js
+import PropTypes from 'prop-types'
 import { Query } from 'react-apollo'
 
 const StarCount = …