Fixed small typo (#952)

Fixed small typo where the closing quotation was missing inside the `fromObject` documentation

src/duration.js

@@ -216,7 +216,7 @@ export default class Duration {
   }
 
   /**
-   * Create a Duration from a JavaScript object with keys like 'years' and 'hours.
+   * Create a Duration from a JavaScript object with keys like 'years' and 'hours'.
    * If this object is empty then a zero milliseconds duration is returned.
    * @param {Object} obj - the object to create the DateTime from
    * @param {number} obj.years