Update reference-react.md (#276)

Hi guys,

I was reading docs on `React.Children.only()` method and saw this statement `Otherwise this method throws.` and it seemed a bit unclear for me what it supposed to throw.

Please let me know if this PR makes sense.

Thank you!

content/docs/reference-react.md

@@ -175,7 +175,7 @@ Returns the total number of components in `children`, equal to the number of tim
 React.Children.only(children)
 ```
 
-Verifies that `children` has only one child (a React element) and returns it. Otherwise this method throws.
+Verifies that `children` has only one child (a React element) and returns it. Otherwise this method throws an error.
 
 > Note:
 >