Highlight all modified lines in the React forms example (#3486)

The change at line 21 was not highlighted, which has been modified with the `handleSubmit` handler added.

content/docs/forms.md

@@ -31,7 +31,7 @@ We can combine the two by making the React state be the "single source of truth"
 
 For example, if we want to make the previous example log the name when it is submitted, we can write the form as a controlled component:
 
-```javascript{4,10-12,24}
+```javascript{4,10-12,21,24}
 class NameForm extends React.Component {
   constructor(props) {
     super(props);